THE VICTORIAN NATURALIST. 109 



the case with some samples from Oamaru, New Zealand, small 

 pieces may be dropped into a warm saturated solution of soda and 

 afterwards dried. The effect of this drying is to recrystallize the 

 soda, and so break up the deposit without injuring the diatoms. 

 In no case must such a deposit be crushed or in any way pul- 

 verized by mechanical force, or the larger diatoms will, of course, 

 be crushed and broken. Some very refractory samples may re- 

 quire boiling in liquor potassge, or other strong alkaline solution, 

 the aim throughout being to prepare the material for the 

 straining processes previously described. These I need not again 

 repeat, the treatment of a recent and fossil deposit from this 

 point being almost identical. The whole process is really more 

 simple and easy than would appear to be the case from my de- 

 scription of it ; and with reasonable care, mainly in the use of 

 sulphuric acid and chlorate of potash, no inconvenience or mishap 

 need be apprehended. I have tried, I think, nearly every published 

 method of any promise, with more or less success, but none have 

 yielded such satisfactory results, though often requiring far more 

 time and labour to carry them through, than the plan I have en- 

 deavoured to describe. There are several matters of detail I 

 should like to touch upon, but must now pass on to a brief con- 

 sideration of the method of mounting diatoms. 



Diatoms may be mounted in various ways — as " spread " or 

 " selected " slides, dry, or put up in some suitable medium. 

 Generally speaking it is better to mount in one or more of the 

 several mediums now in use, and whenever diatoms are to be 

 mounted for examination with immersion lenses this is imperative. 

 I will first of all refer to the several media in general use. The 

 oldest, and I might say the best medium but for its low index of 

 refraction, is Canada balsam. Mr. Morland, an authority on 

 matters of this sort, says that " for the coarser kinds, say four out 

 of five species at the very least, Canada balsam, taken all round, 

 remains as good as any other medium yet brought forward." 

 Next to Canada balsam styrax may be regarded as a suitable 

 mounting medium. Its refractive index being considerably higher 

 than that of Canada balsam, renders it serviceable where it is 

 desirable to bring out difficult or obscure points of structure. Its 

 chief drawbacks are that it is usually very dark in colour and re- 

 quires a long time to harden. American styrax is said to be free 

 from these disadvantages, but I have had no experience of it, nor 

 have I seen it used. Monobromide of naphthaline has a refractive 

 index higher than either of the foregoing, but being a very fluid 

 oil it requires care and skill to secure it permanently. But a 

 mixture of this medium with Canada balsam is in my opinion the 

 best medium I have tried. It will stand almost any degree of heat 

 without the formation of bubbles, and is not discoloured thereby. 

 It possesses great brilliancy, and the further advantage of a higher 



