76 PREPARATION OF DIATOMS. 



" This dry deposit is then put into a phial and twice its volume of 

 concentrated sulphuric acid poured on it. This is allowed to act for 

 several hours, during which it is frequently shaken. The mass will grow 

 black. Guanos require from five to six times their volume of sulphuric 

 acid. This acid is the only efficient solvent of excrementary debris, and 

 even then most of it can be removed by decanting three-fourths of the 

 sulphuric liquid after leaving it to rest for a sufficient time. On this 

 thick gruel-like and blackened matter some coarsely-powdered potassium 

 bichromate is then added. It should be added by small successive 

 doses, giving it a good shake each time. The mass becomes heated 

 and oxygen is often given off. It should be stopped as soon as its 

 colour has changed from black to red, or when crystals of chromic 

 acid have been formed. In this treatment the organic matter is 

 carbonized by the sulphuric acid, and the nascent chromic acid effects 

 its combustion. The preliminary washing with hydrochloric acid is 



made so as to avoid the formation of lime sulphate. 



" To the above liquid is added water little by little. The mass again 

 becomes heated. An abundant supply of water is then added. The 

 colour of the resulting deposit is now comparatively white. It is then 

 carefully washed by decanting. The last decantings are made with dis- 

 tilled water. It is then ready for use. For this purpose it is diluted 

 with distilled water, and the mixture dropped on to large cover-glasses, 

 on which it is dried. From these cover-glasses the selection of species 

 is made." 



Preparations with Styrax and Liquidambar. Towards the middle of 

 the year 1883 I disclosed a new method of preparation, which I had 

 myself practised for some considerable time. 



I have given up Canada Balsam and I use Styrax instead, which, 

 while being easier to manage, has a considerably greater index of 

 refraction and shows the details of diatoms much more clearly. Since 

 I introduced this medium to the public I have had the satisfaction of 

 seeing it adopted by the most competent diatomists. I will therefore 

 shortly explain the method of preparing and using Styrax. Styrax is a 

 natural balsam which exudes from the Styrax Orientalis Miller, a native 

 of Asia Minor. 



It must be bought in the raw state as sold commercially, when it 

 appears in the form of a soft, greyish mass. A thin layer of it should 

 be spread on a plate and exposed to the air and light until it becomes 

 sufficiently hard and has lost all the water which it contained. 



This is then dissolved in a mixture of equal parts of alcohol and 



