110 H. NORLAND ON MOUNTING MEDIA FOR DIATOMS. 



any other medium yet brought forward. Although balsam is 

 comparal Lvely colourless in its natural state, it gets to be a very 

 deep yellow when hardened by means of heat; this colour is, 

 however, scarcely noticeable when in a very thin layer under a 

 covering-glass. If over-hardened, balsam becomes brittle, and 

 the cover is easily detached by a blow or rough usage ; on the 

 other hand, if under-hardened, not only w r ill the slide be too 

 tender to handle, but the refractive index will be considerably 

 lower than should be the case. ^ 



Styrax. — This is a medium about which a deal of misconcep- 

 tion exists. As a matter of fact, no styrax, so far as I am aware, 

 has yet been used as a mounting medium for diatoms or any- 

 thing else ; but I must explain. The true styrax is a product of 

 styrax officinale, belonging to the order Styracece. It is a 

 native of Greece, the Levant, and Asia Minor, and was the 

 source of the original and classical storax. It has, however, 

 now wholly disappeared from commerce, and I much doubt if it 

 be obtainable in this country, its place, so far as our British 

 Pharmacopoeia is concerned, being now supplied by the product 

 of Liquidamber orientate, belonging to a totally different order, 

 viz., the Altingiaceae. This substitute can be purchased at any 

 chemist's. When getting it " strained styrax " or " styrax 

 colat " must be asked for. It has a strong resemblance to very 

 thick treacle, is full of fine dirt, and, I think I may add, of 

 moisture also. There is yet another false styrax, sometimes 

 called " American styrax," which is the product of " Liquid- 

 amber styraciflua " or " sweet gum," a tree found in the 

 Southern States of America, mostly, I believe, in damp situa- 

 tions. This latter medium is not, however, obtainable in 

 England, and it is only comparatively recently that I have been 

 able to ascertain where it could be obtained in the States, as 

 even in its native country it is difficult to get hold of. I shall 

 be happy to give the address to anyone desiring the same. 

 Like the other false st}Tax, this American styrax is also sup- 

 plied in a rather dirty state. As the true styrax has no 

 existence so far as we are concerned, I shall now speak of the 

 other two media simply as " styrax " and " American styrax." 

 Both the styraces, which were first introduced by Dr. H. van 

 Heurck, of Antwerp, can be dissolved in either benzole or 

 chloroform and afterwards filtered. They are apt to deposit a 



