The Microscope. 173 



sloping sides. Provide, also, a number of glass squares sufficiently 

 large to serve as covers for the dishes. 



SIMPLE MOUNTING AND STAINING. 



In this lesson will be discussed the art of making simple and 

 permanent mounts. As the method to be here described will apply 

 to all balsam mounts, however complicated the preparation of the 

 specimen may have been, the reader will do well to learn thoroughly 

 the rationale and order of the various stages employed. 



From a cat or other animal obtain a piece of mesentery. Wash 

 carefully in distilled water, and then lay it in one of the dishes and 

 cover with the silver-nitrate solution. Leave it there for about two 

 minutes, then remove and wash again. Now place it in a salt- cellar 

 full of distilled water, cover with a glass plate and expose to the 

 light — not direct sunlight, however — until it assumes a light-brown 

 coloi". This usually takes a number of hours, during which time the 

 water may be changed a few times. The color appearing, the mem- 

 brane will then have been sufficiently stained to show the outlines of 

 the endothelial cells found on its surface. This result will have 

 been brought about by the fact that the silver salt combines with the 

 intercellular substance to form an albuminate, which, white at first, 

 dai'kens on exposure. 



The specimen is now to be put through the following course: 

 (a.) Immersed in alcohol; (6.) transferred to oil of cloves or tui'- 

 pentine ; (c.) mounted in chloroform-balsam. 



The rationale of all this is: The specimen is treated with oil of 

 cloves or other essential oils, as origanum, cedar, etc., or with turpen- 

 tine, to render it more transparent, these substances possessing great 

 refractive powers. If the reader wishes to get a clear idea of the 

 transparency thus induced, let him put a piece of unstained mesen- 

 tery into the oil — of course, after immersion in alcohol — and he will 

 notice that it seems to melt away as it becomes permeated with the 

 oil, and finally to disappear altogether. 



Before the specimen can be put in the oil it must, of course, be 

 freed from the water it obtained when being washed. Alcohol, 

 having a great affinity for water, is employed for this purpose. 

 But to do this the alcohol must itself be comparatively anhydrous. 

 Absolute alcohol, which contains not more than .2 per cent, of water, 

 is the surest, but that containing not more than 3 per cent, will 

 answer if. the specimen be allowed to remain in it for a longer time. 



So important is this process of dehydration — for on it depends 

 the futui"e preservation and transparency of the object — that it must 

 be carefully performed. 



