phere at the time of making the fillings, and then subjected them to varying degrees of humidity in an incubator which was running at body temperature, instead of allowing them to stand in the room at room temperature and humidity. In some of these tests enough expansion had taken place so that we could see it. The same tests were then carried out by placing the fillings made under office conditions under water at body temperature and allowing them to set. In these tests we noted an expansion with the naked eye tho they differed from the ones placed in the incubator by being disintegrated on the surface apparently from over hydration. In both these series of tests we eliminated the use of the micrometer on account of the damage that might be done to it by the wat

In

These earlier readings showed that we had missed the first movement of these cements entirely for they generally showed expansion as the first movement, and then under some conditions, principally when a thin mix was made or the humidity was too low, a shrinkage. one case we recorded an expansion of 185 points on the micrometer (.0185 of an inch on a filling .250 of an inch high) inside of three minutes, and then a shrinkage of 13 points during the next eighteen hours. This test, however, was one of a series that was made to purposely get a large amount of expansion. For it we used 1.700 grams of light yellow Petroid powder and 0.98) grams of rapid Petroid liquid, added the powder in

three portions, and placed it in the incubator running at body temperature and having a humidity of saturation. The majority of these tests ran from forty to sixty-five points in the first three minutes and some times inside of 20 seconds and then a shrinkage of from two to four points in the next eighteen hours took place. Since in most of these fillings there was only one side of it exposed to moisture, and since those fillings that we subjected to moisture after they were made showed less shrinkage as the second movement, and since the work done on the cement during the mixing seems to vary the amount of the movement, and since the temperature at which the liquid and powder are brought together seems to be a factor of much importance in determining the reaction that takes place, we are uncertain whether shrinkage should be regarded as the second movement that takes place in these products when other conditions prevail. We have not made a thin mix such as is used for the setting of inlays, however, that did not show shrinkage as the second movement and in most cases it was quite marked. This experience in dealing with but one property of but one of the different cements made by this one concern served to point out that we were dealing with a reaction that was subject to the slightest variations in temperature, humidity, condition of contact of the reacting substances, agitation of the combining substances, etc. In fact, we regard the determination of this one property on the various cements now in the market an enormous piece of research in itself. As soon as we came to this conclusion we abandoned our attempt to determine the various properties of the market products because the problem appeared more complicated as we found from our analyses that were going on that the principle differences in these cements in the market was in the powder portion, the liquid portion being synthesized along the same lines, viz., hy

drated aluminum oxide, and water added in small quantities, to orthophosphoric acid. It appeared to us that if the determination of one property would furnish a research in itself we had better determine what constituent or constitu

of impurities from the utensils used in its manufacture such as ball mills in grinding, crucibles in calcining, etc. Ames' Pearl White we found to contain principally zinc oxide, modified with a little less than one per cent. of bismuth,

and ferric-oxide which we interpreted to be contamination from crucibles, ball mills, etc. To verify our analyses we began syntheses of these powders beginning with the two that appeared to contain zinc oxide only.

From the literature on the subject we had constant reference to zinc oxide being highly calcined for cement powders, so after trying both Justi's and Fellowship original liquids, and our synthesized liquid from the analyses of these originals, with Kahlbamn's, and Schuchardt's, and Baker's zinc oxide with a guaranteed analysis for impurities we began to heat these different samples of zinc oxide in both the electric and gas furnaces.

These different powders when uncalcined and mixed with the liquids suggested made cements not suitable for dental purposes. The finer and lighter the zinc oxide the more rapidly they appeared to set forming little nodules apparently due to the rapidity of the reaction.

We began the calcination of the Baker's product because we were not certain that if the supply of Kahlbamn's and Schuchardt's zinc oxide should become exhausted during the experiments that we could replenish it on account of the conditions existing abroad at the time.

Our first sample was placed in a platinum crucible and covered with the platinum cover that accompanied the crucible, and placed in the electric furnace and the temperature raised to 1000°C from a cold furnace inside of 30 minutes, and then this temperature held for 30 minutes, after which the crucible was removed and allowed to cool. The zinc oxide showed a considerable loss from the ignition, probably mostly from dehydration. It is possible, however, that even this short time at this temperature would cause some of the zinc oxide to volatilize. Schnabel quotes Stahlschmidt as saying that zinc oxide is volatile at 970°C, and that at 1054°C it is about 15%. A further possibility of

some of the loss on ignition being due to other things than dehydration is shown in our experiments, that will be mentioned later, that show that zinc oxide is reduced to metallic zinc to some extent while firing, and metallic zinc is known to be volatile at a point somewhat lower than the zinc oxide is. The working properties of the zinc oxide from this amount of calcination were changed but little. Another sample was given the same treatment except that the temperature was maintained at 1000°C for an hour instead of thirty minutes as before. Little change in the working properties of this zinc oxide was noted. A third sample was given the same treatment except that the temperature was maintained at 1000°C for two hours. This zinc oxide was little different than the others.

As we were about to clean the crucible for the fourth sample a hole appeared in the side of it and upon close inspection it was found that a defective ring extended nearly all the way around it. We then went to the Department of Chemistry and purchased some pure Magnesium Oxide crucibles that we knew were made for some similar work some years before, and continued to maintain the temperature of the furnace an hour longer each time we fired the zinc oxide. On the fourth firing our pyrometer failed to register the temperature. Inspection revealed that the thermo-couple which was being held near to the crucible in which the zinc oxide was, had fallen to the floor of the furnace in several pieces. which appeared like a dark colored powder. This convinced us that platinum or its alloys with iridium or rhodium could not be used either for crucibles or thermo-couples unless some form of protection could be devised which would keep them from what appears to us as either volatilized zinc, or zinc oxide. This proved somewhat of an obstacle because we had but one pyrometer available at the time. We purchased another

Siemen's & Halkse and calibrated it with the one used as a standard in the Department of Chemistry and used our original one to experiment with. We purchased a thermo-couple from the Pelton & Crane of Detroit covered with a metal cover that was said to be one used in connection with the tempering of tools in hot lead baths and asked Mr. Crane of the Pelton & Crane Co. to come to Ann Arbor and witness the making of a scale and the calibration of the pyrometer that we were to use for experimentation. This done and a new Siemens & Halske instrument already connected to check the one with the covered couple we thought we might go on with our study of the amount of calcination to be given zinc oxide to make a good cement. At the end of the six or seven firings the metal cover (said to be iron) fell to the floor of the furnace in small pieces. This furnished another source of trouble. We had a standard pyrometer already attached this time, however, with which to construct a working instrument. This time we went to the Eberbach & Son Co. of this city and purchased a piece of Royal Berlin porcelain tubing with a 3 inch hole in it and arranged the carbons in the reflectoscope in the lecture room so that they made a good sized arc and fused one end of this tube together. We then had made up a thermo-couple about ten inches long and placed inside this tube, and made another scale for the instrument. Using this instrument for the work and the newer instrument for a check has been very satisfactory and will in our opinion prove the most satisfactory way for future workers on the subject of zinc or zinc oxide, or similar substances to determine the temperature. One thing seems certain, viz., that either zinc oxide or zinc volatilize sufficiently during the calcination process to destroy metal thermo-couples and crucibles. One thing that leads us to believe that it is not zinc oxide alone that makes the trouble is that often times the

zinc oxide in the crucible takes on a greenish color around the outside of the mass next to the crucible and some distance beneath the surface showing apparently that there has not been a free enough passage of oxygen in the crucible in the furnace to keep the zinc in oxide form. This is especially noticeable when the gas furnace is used and a poor adjustment of the air and gas exists.

It was soon found that the electric furnace was not going to stand the temperatures that appeared necessary to calcine the zinc oxide to a point where it would make a good cement so we continued the work in the Meeker gas furnace. This furnace had a capacity of slightly over 1400°C tho it was supposed to be capable of 1800°C. In it we made up samples of calcined zinc oxide from Baker's dry process product with an analysis as follows for impurities: Fe..002 Pb..05

Cd. trace C1 .01 SO3.05

These samples were all made in the magnesium oxide crucibles referred to at 1400°C for from one hour to fifty hours in the furnace. In each case the zinc oxide came out of the crucible in one solid piece shrunken to a small sized piece and quite hard with the longer firings. We then took up the question of grinding these apparently fused cakes of zinc oxide and settled upon five by seven inch Royal Berlin porcelain ball mills containing not less than 25 Royal Berlin porcelain balls and revolving at 68 revolutions per minute the equipment. As we observed the effect of these mills in grinding and the appearance of the powder portion of Justi's and Fellowship cements under the microscope and by measuring the particles with the micrometer on one of the microscopes we found that it took from fifteen to twenty hours to get some of

as

the samples that had been fired for a long time to have the appearance of Justi's powder. Then came the question of getting a powder that had particles of somewhere the same size. For this purpose we purchased several grades of bolting cloth some running as fine as 200 mesh to the inch. We soon arrived at the size of the powder particles that resembled Justi's and Fellowship powders but to determine which of the various grades of firing from one to fifty hours in the furnace at 1400°C, would react with the liquid for these cements caused us to do some careful work in mixing under conditions that could be duplicated. Finally, we settled upon the powder that had been fired at 1400°C for 14 hours at the one having the properties nearest to Justi's. It proved to be so close that we could substitute our synthesized product for the Justi powder in skilled operators hands without its being detected. Our work this far seemed very satisfactory as far as the results were concerned, tho we had encountered some obstacles.

We then went over the Fellowship powder in the same manner but could not select a single powder from the lot we had made that seemed to come near duplicating it. We then began mixing various grades of the zinc oxide and finally decided that a mixture of 15 grams of zinc oxide that had been fired 14 hours at 1400°C, and 8 grams that had been fired 8 hours when the two had been mixed in the ball mill for about ten hours was very near to the original powder. We had observed that the longer zinc oxide was fired the slower it reacted with the cement liquids, that they were very adhesive and expanded slightly, that the zinc-oxide took on a yellow color at about six hours at 1400°C which became deeper as we approached the samples fired at 50 hours, that at about eight hours the zinc oxide in the crucible began to clinker into one solid cake, that the broken and ground mass had a

heavy glassy appearance, and that the reaction between zinc oxide and cement liquids decreased with the calcination, tho not very marked after passing that which had been fired for 15 hours at 1400°C. It appeared to us at this time that a cement composed of zinc oxide alone would be limited in color to the slightly yellowish white of the zinc oxide that had been fired at 1400°C for four hours or the deeper shades of yellow that followed the higher calcinations, the unfired white zinc oxide appearing to be too rapid in setting for use even in small quantities. We had observed that the Ames' and Caulk and some other products had a variety of colors, principally yellows and whites. The brown and yellow crown and bridge powders we found to be tinted with a small percentage of Ferric Oxide.

There was some question whether even the yellowish white and yellow colors could be produced from a straight zinc oxide powder since we had statements in the literature that we interpreted to be conflicting in regard to zinc oxide turning yellow when it was heated. An authority like Ostwald says:

"Zinc oxide is white in the cold, but appears yellow when hot; on cooling it again acquires a white color. This color change must not be regarded as a sign of the conversion of the zinc oxide into another, perhaps allotropic, condition, for it does not take place suddenly, as in such a case it would do. but gradually. It is solely due to the fact that the region in which zinc oxide absorbs rays moves, on heating, from the ultra-violet portion of the spectrum, in which it is situated at the ordinary temperature, towards the visible violet portion. This is a very general phenomenon, viz., that the region of absorption of rays changes in the above sense with the temperature. White substances become yellow on being heated, yellow ones red, and red ones brown; blue and green substances, on the other hand,