been modified with alkaline phosphates or borax.

Class 2 consists of cements resulting from a mixture of some form of zinc oxide per se; zinc oxide bound to or admixed with tin oxides, silica or magnesia, or any two of these or all three; or with alumina per se, or alumina bound to or admixed with lime or silica or both, with orthophosphoric acid that has been modified with zinc phosphate or aluminum phosphate or both.

Cements of class one are practically worthless. At times their liquids contain boric acid in large quantity. In such cases sodium phosphate is an invariable accompaniment, leading to the conclusion that borax has been added. No benefit results. As an antiseptic the resulting boric acid is perhaps inferior to the zinc salts present and both boric acid and sodium phosphate are soluable in water, rendering a cement containing them subject to disintegration by the oral fluids. When, on the other hand, acid phosphates such as exist in the liquids of the second class have reached complete chemical union with their powders, there results the maximum insolubility ever to be hoped for in plastics of the phosphate type."

In discussing the powders of the cements Dr. Fleck says, "Zinc oxide of commerce is the basis of cement powders. By a process of purification the zinc carbonate formed is calcined to a light oxide. So prepared, the material is entirely unsuitable for plastic purposes, possessing great shrinkage, developing much heat and setting far too rapidly with even highly modified liquids." "Density is the factor missing. The required conditions are reached by treatment with nitric acid, evaporating and subsequent calcining in suitable vessels; by fusion with certain fluxes, like borax, and subsequent solution of the flux; by fusion of zinc sulphate with sodium sulphate and subsequent lixiviation; and by prolonged calcining at very high heat

By

in specially constructed furnaces. these processes the specific gravity of zinc oxide rises from 5.42 to 6.20 and the solubility in acid is vastly decreased."

On account of the arrangement of the article it is somewhat difficult to quote briefly the matter contained in Dr. Ames' paper previously referred to. Some of the matter that is worthy of note in this connection follows, viz.: Justi's cement has held a place in dental procedures for more than 30 years, and must therefore, be possessed of unusual merit. This claim, however, that the formula of this cement has not been changed in 37 years, I do not consider as entirely commendable. In fact the absence of crystals in the liquid as furnished now for some years, indicates a change, as the early product usually contained precipitates. Its producer was one of the earliest to depart from the sodium phosphate formula." These cements, kept dry for a sufficient time for setting before being subjected to moisture, give a mass which is slightly porous, shows exceedingly well in adhesion tests, and shows a barely perceptible expansion. These desirable qualities, expansion practically nil and exceedingly good adhesiveness, are dependent on the liquid element, the powder being rather coarse heavy zinc oxide." "Crowns set with this material thirty years ago are in acceptable condition today, as far as the cement attachment is concerned, and gold inlays of the burnished matrix type twenty years in service bear evidence of the reliability of this cement. However, when we became impressed with the practicability of the cemented metallic inlay and the porcelain inlay as made twenty years since under selected conditions there arose a seeming need of a cement combining a finer and smoother powder, a somewhat quicker setting, and, if practicable, a cement with what we are in the habit of calling "hydraulic" properties."

It may be noted that Dr. Ames says

that Justi's cement is not made from a sodium phosphate formula, and he implies that none of the other makers are using one. In fact later in the article he says that makers of reliable cements depend upon non-alkaline modifiers of the liquid. Dr. Fleck in the article referred to said that "cements resulting from a mixture of some form of zinc oxide with orthophosphoric acid that has been modified with alkaline phosphates or borax are practically worthless."

f

Mr. Paul Poetschke, in the article referred to, in discussing the germicidal efficiency of dental cements, especially those containing copper, says "the liquid supplied with a majority of these cements consists essentially of concentrated solutions of orthophosphoric acid, H3 PO4, modified by the addition of hydrated oxide of alumina, A12 (OH)6, occasionally other metallic salts such as iron and nickel are added." Here we have statements from two manufacturers dental cements that imply strongly that alkaline modifiers of the liquid, such as sodium phosphate, have been practically abandoned, and the third frankly states that the acid phosphate, hydrated oxide of aluminum, A12 (OH)6, is the principal modifier now in use. These statements from those in a position to know what is being used seem to furnish sufficient evidence, that our suspicions that the greater part of the text-book literature was obsolete, were well founded as far as the liquid portion was concerned. As may be seen later our analyses confirm the statement of Mr. Poetschke and the references to the subject by Drs. Ames and Fleck.

It may also be noted that Dr. Ames makes reference to his observing some years ago that the wide use of both the gold and porcelain inlay was calling for a finer and smoother powder, a quicker setting cement, and one with the socalled "hydraulic properties." While he does not say so in as many words, Dr. Ames implies that his powder has been

changed from a straight zinc oxide, with some of the properties he has attributed to Justi's cement, to one with a finer and smoother powder, a quicker setting cement, and one with more hydraulic properties.

In the same article he presents cuts showing highly magnified specimens of the crystals used in his inlay cement before it is ground, and the same of the crystals used for his crown and bridge cement, and follows by saying that ordinary oxide of zinc, either before or after vitrification, presents no crystallin appearance whatever. Mr. Poetschke admits the use of oxide of bismuth and magnesia as modifiers of the powder and it is with much concern that the word bismuth is noted here because as we will see later oxide of bismuth appears to be the material that makes the Caulk and Ames cements, and possibly one other, differ from all others that are sold thruout the Middle West, not that oxide of bismuth has cement making properties in itself but because it appears to be a material that will so modify zinc oxide that the resulting product has many of the properties that Dr. Ames claims for his latest efforts to meet the demands of the inlay workers. Our analyses show that both Caulk's and Ames cements contain bismuth, and we suspect that Smith's cements have it too, tho this suspicion is based on its working qualities only, no analyses having been made. The article by Dr. Fleck does not give a hint that bismuth is a constituent of the cement powders of the most recent type, but since this article was written in 1902 it seems doubtful that the use of bismuth by the makers of the more recent products was known by him at that time. In another article by him in Items for 1908 he mentions bismuth and suggests its being added as a sedative.

With these statements of Ames and the one by Poetschke which did not reach us until April first, 1915, and the

analyses that we will give later, and the confirmation of these analyses as far as they have been carried out by a synthesis of the product represented by the analysis, it seems that the references to the powder portion of dental cements in practically all our text books are of little value, first because they give little or no information relative to the great variety of forms of zinc oxide that may be prepared for those cement powders that are zinc oxide only, and second because no mention is made of the modifications that have recently been developed in zinc oxide with bismuth, or bismuth and magnesia to form these powders for the more recent cements.

Reference to the condition of the dental text books on the subject of dental cements is made at some length with a view of aiding future workers rather than to criticise those who offered the articles referred to including the one by the writer in the American Text Book of Operative Dentistry. In the earlier part of the year we could hardly believe our analyses right because they did not show some of the ingredients mentioned by almost all the text-books in which the subject was mentioned. The result was that we went over the analyses oftentimes three or four times to be sure that the text-book was wrong. And, to further confirm our analyses showing the textbook wrong it was necessary to do days, and in one or two instances weeks of work that could have been avoided if we had ignored the entire list of text-books and confined our work on the available literature on the subject to an effort to disentangle some of the references made in the articles by the three manufacturers mentioned. Dr. Ames had said in the article previously referred to that "Information obtainable from dental chemistries, dental formularies, and magazine articles, generally seems to be

based on tradition or insufficient investigation," and we would perhaps have been inclined to regard the warning with favor had we not had the feeling expressed by Dr. Fleck in the article referred to, that "Facts, never revealed to light, lie in the hands of manufacturers who, standing aloof from all the general ideas of those whose demands created them, jealously guard their discoveries, yea, even the groundwork thereof, for commercial reasons."

Since that time, however, Dr. Fleck has entered the manufacturing field and it will be interesting to note the information he contributes now.

After getting the specific gravity of the liquids that we had on hand that had accompanied Petroid powder we used more careful technic in getting the hydrated aluminum oxide into solution and made a water correction to the specific gravity of the Petroid liquid under examination. This synthesized liquid appeared so close to the liquid that had accompanied the powder that we substituted it in the hands of some of the most skilled operators we knew and none could distinguish the original from the synthesized product. We concluded from this that our analysis was approximately correct and that our technic in synthesis was at least close enough so that it could with further study, and the manufacture of larger quantities be made very good. We followed this examination of the liquid of Petroid cement, with a similar one of Ames C liquid, the one that, in our opinion came nearest to the setting qualities of Petroid rapid setting, also Fellowship liquid, and Justi's liquid. While slight differences appeared it seemed that they were very similar in the modifiers, hydrated aluminum oxide and water, in quantity. Justi's liquid appeared a little slower and Fellowship a little faster in setting than our synthesized liquid.

We then undertook a series of analyses of the liquids of the Ames' and Caulk's cements to determine what, besides the percentage of water determined the setting time of these products. This proved somewhat unsatisfactory in one respect, viz., that we did not find the differences in quantity of hydrated aluminum phosphate between a slow setting and a rapid setting cement that we had expected. In all cases, however, we found a higher concentration of the

aluminum salt in the slower setting liquids.

Had we not become so much occupied by one or two other problems that will be mentioned later we would have gone over the same liquids again to perfect our technic and get analyses for this report. This, however, will be taken up another year. Since we had interpreted the small amount of hydrochlorid acid to be in the nature of an impurity from the aluminum salt or other source the composition of the liquid appeared to be orthophosphoric acid, hydrated aluminum phosphate, and water. Since the literature had so many references to the use of sodium phosphate in the liquid we made careful examinations for this salt in the liquids in the market and added small quantities in our synthesized liquid to note the effect, but we were convinced that if it were used at all it was in quantities scarcely discernable. Our opinion is at this time that the uncertain tests that can be obtained for sodium in some of these liquids is due to the presence of this salt as an impurity from the utensils used in the synthesis of the liquid. As we get to a closer observation of the properties of these products we may find, however, that it is purposely added in very small quantities in one or two of the liquids mentioned. As these analyses and syntheses were being carried on we undertook at odd times to determine the various properties of these cements other than the specific gravity and setting that we thought would be of the most interest to the dentists generally, and to get a reasonably good survey of the character of the powder portion. We had found the liquid portion of these products to be essentially alike and expected to find the powder much the same tho we could detect such differences in the properties of Justi's Fellowship, Ames', and Petroid cements that there remained a feeling that, if this were true, the manufacture

of these powders was subject to great variations.

We found in the four cements mentioned four classes of cements as far as the modifiers of the powder portion is concerned, and with every variation in the character or quantity of modifier we found a different cement to handle. Two of the most conspicuous differences were, the property of setting promptly in the presence of moisture and smooth working. In these two features we found Ames' cements, and Petroid to excel the other two. In resistance to abrasion and the insolubility tests that we subjected them to the Justi and Fellowship seemed to be very good. An examination of the powders under various magnifications showed the Ames' and Caulk's products to be a little finer than Justi's and Fellowship, the Justi's product being the coarsest of the ones mentioned. Some tests were made for adhesiveness, crushing resistance and volume change. These provided the most extraordinary surprises that we had had in our work thus far. We referred to the literature for suggestions on manipulation, humidity, etc., but found little of value. In one or two instances we tried to confirm the tests that had been reported in the current literature relative to the behavior of some of the products in the markets but were quite generally unable to do so.

When we began to summarize our tests for adhesiveness, crushing resistance, and volume change we found much of our data very conflicting in nature and generally unsatisfactory. Some of this unsatisfactory data later proved to be the result of faulty handling on our part. The remainder may be cleared up by future work. An example of this which caused us considerable embarrassment was found in trying to determine the volume change in the four cements mentioned. We were employing a special micrometer that was designed some years ago by the

writer for determining volume change in plastics (See American Text Book of Operative dentistry, Page 295), the ordinary test of packing cement into short glass tubes and placing some colored solution over one end when the cement was set, and the picnometer for determining the volume changes in these cements. With Ames' and Caulk's rapid setting cements our data showed quite general shrinkages, tho there were variations in the amounts. We concluded from this that we were in error in our manipulation and at once invited some of the most skilled technicians in our acquaintance to make the tests for us and the result was the same. In some cases a shrinkage that would let the filling fall out of the receptacle holding it when turned on end took place. The matter was referred to a representative of one of the companies mentioned who was demonstrating the use of cements and he promised to reply to the matter later. The question asked was "do the makers of these cements so compound them that they shrink with the ordinary manipulation given them." We assured him that we were satisfied with the cement from a practical standpoint as evidenced by our use of it in the college clinic, and asked for an opinion from his makers from the standpoint of knowing. He understood that the work we were doing was sanctioned by the Research Commission of The National Dental Association, and should have known that we would find out eventually. Later his reply was that he was not in a position to give out information relative to his company's product. Still we believed that Ames' and Caulk's cements shrank because in some cases it was so much that we could see it. Our experience up to this time had been with fillings that were made and kept under ordinary office conditions. We then went over the same tests but made them under office humidity and made a note of the temperature of the slab and spatula and atmos