UXPRESSED MOUNTING FOR MICROSCOPE. 131 



things, air diffused through an object is the most ///-transparent, — • 

 difhcult to get rid of, and misleading in its appearances to the 

 microscopist. Most bodies contain about seventy per cent, of 

 water ; and in drying an object, therefore, we get rid of all this, 

 partly by shrivelling up the object, partly by replacing the water 

 with air. Then, having spent some time and effort to get the 

 object well filled with air and nicely shrunken up, we set to work 

 with still greater trouble to get the air out again, and to puff out 

 the specimen to something like its former shape. Hence, whatever 

 else we do, we will not dry our object. That part of the tissue of 

 the blow-fly's head which is not swollen with water is filled with 

 air ; and so, w^hile taking out the colouring matter, it will be an 

 economy of time to get rid also of some of the air. What 

 apparatus do we need for this ? would not an air-pump be of use ? 

 J3y all means, if you can afford it, and if you wish to add another 

 to your array of instruments, go and purchase an air-pump, and do 

 whatever you like with it, only do not use it for microscopical 

 mounting. Go, instead, and buy a half-penny test-tube ; for a 

 solitary test-tube is the whole of the preparing apparatus needed in 

 this method ! 



Into this test-tube place the fly's head, and fill the tube half- 

 full with a solution of soda or potash. Stand the tube in a cup or 

 tin pot of boiling water, and leave it on the hob of a fire or other 

 warm place to keep hot till morning. Then examine the head 

 and see if it looks almost transparent ; if not, pour off the soda- 

 solution, and add a fresh supply, and again keep the tube hot till the 

 object becomes semi-transparent. Now pour off the solution and 

 add hot water, in a few minutes emptying it out and adding some 

 more : — repeat this at least three times, and finally leave the last 

 quantity of water on the object for an hour to cool. Next pour 

 off all the water and replace it with spirit of wine ; methylated 

 spirit, if strong, will do sufficiently well. Heat this by immersing 

 the tube in a vessel of hot water for one minute ; then take it out, 

 cork it up, and leave it for one hour. 



So far, we have, by means of the soda-solution, destroyed all the 

 flesh and fat-tissues, leaving only the cuticle and internal organs, 

 such as the tracheae, etc. In doing this, we have filled up most of 

 the few natural air-spaces with soda-solution ; which, however, 

 being a somewhat dense fluid, would not enter many of the 

 narrow tracheal tubes. Then with water we replaced the soda- 

 solution, and washed away the parts destroyed thereby. On 

 replacing the water by alcohol, — -a still less dense fluid, — more of 

 the finer air-spaces are penetrated and the air driven out : there 

 are still, however, some tubes too minute even for alcohol rapidly 

 to enter. So now we pour off the spirit, and add ether instead, 



