The Microscope. 47 



he has used the cement for man}^ j^ears, and having made it him- 

 self with great care and attention to the details, he has had with 

 it perfect success, and has every confidence in it. I have never 

 seen any of Dr. James' mounts, made with his own white zinc 

 cement, but I accept his statements as coming from one having 

 authority, and the reader, if he can obtain the cement, as made 

 by Dr. James, ma}^, I am sure, use it with every confidence. 

 The trouble seems to be that Dr. James' cement is not in the 

 market ; that used and for sale by the dealers is as abominable 

 as anj'thing can be imagined to be. 



The cement is commonly made b}^ dissolving pulverized o-um 

 dammar and gum mastic in benzol ; to this solution is carefully 

 added the white zinc (zinc oxide), rubbing it long and faithfully 

 in a mortar. It is then filtered. Dr. James, writing about the 

 proper method, says : 



Three points are absolutely essential to success in manufactur- 

 ing a good article of the oxide of zinc cement : good materials, 

 time and patience. The zinc white must be white, not 3'ellow 

 and must be thoroughly free from moisture ; the dammar free 

 from dirt, and the benzol of the first quality. The solution of 

 dammar in benzol should be thin enough to admit of filterino- it 

 through absorbent cotton or mineral wool, though if the o-um 

 be carefully picked it will be sufficient to run it through two 

 thicknesses of an old Japanese or Chinese silk handkerchief. A 

 prime essential to succsss is to rub up but a little of the zinc 

 and dammar at a time. Not over a dram of each should be put 

 into the mortar together, and trituration should be continued 

 till all grittiness disappears. The author formerly filtered the 

 finished product through silk or linen, but the task was tedious 

 and the waste large. He now simply agitates the product, and 

 sets it aside for twenty-four hours. At the end of this time he 

 decants the supernatant portion and returns the sediment to 

 the mortar, again triturates with a fresh portion of the dammar 

 solution, and repeats the operation of decanting, etc., until a 

 perfectly smooth, homogeneous cement is obtained. If, in the 

 end, after allowing the cement to stand in a perfectly quiet 

 place until separation occurs (this will take at least six weeks, 

 if the cement is thoroughly well made), it is found that the 

 liquid is much in excess of the solid portion, the excess may be 

 decanted and used for making more cement, or any other pur- 



