INTRODUCTION. XXXI 



portions of talc, which can at any time be placed under the 

 microscope, and examined without further preparation ; and this 

 mode possesses one great advantage, that the specimens can be 

 submitted without fm'ther preparation to a heat sufficient to 

 remove all the cell-contents and softer parts, leaving the siliceous 

 epiderm in a transparent state. But this method will not preserve 

 the natural appearance of the filamentous, stipitate, or frondose 

 forms ; nor will it satisfy the amateur who desires a specimen 

 for his cabinet, and demands a mounting of a permanent kind 

 and neat appearance. 



To meet these requirements, it is necessary that the specimen 

 should be preserved in fluid or balsam. The modes of mounting 

 objects in both these ways are best learnt by the examination of 

 specimens thus prepared; and the manipulation required can 

 only be successfully acquired by practice. 



I have never found written or verbal directions of much real 

 value, and shall confine myself to a few hints, which may sup- 

 plement the discernment and patience of the operator, but can- 

 not supersede their presence nor supply then* absence. 



The walls of the artificial cell for the filamentous and stipitate 

 forms, I invariably form of litharge (protoxide of lead) and japan- 

 ners' gold-size ; and I attach the thin glass covers with a cement 

 of gold-size, mixing with it a Httle lamp-black, to give a darker 

 colour to the last coating applied. 



The fluid I employ is distilled water, without any mixture 

 whatever, having found all compound fluids not merely unneces- 

 sary, but injurious. 



If the filamentous and stipitate forms are not, however, 

 mounted in a fresh state, the frustules separate from each other, 

 part from their stipes, and lose their characteristic appearance. 

 To remedy these inconveniences, I immerse such specimens as 

 cannot be placed in cells when freshly gathered, in spirits of wine 

 and water — one part of the former to six of the latter, and their 

 attachment to their stipes remains afterwards undisturbed, unless 

 violence be employed to separate them. 



In preserving the Diatomaceae in balsam, the siliceous valves 

 are the portions alone required. Indeed, it would be imprac- 

 ticable satisfactorily to mount specimens in their natural state 



