TECHNOLOGY OF DIATOMS. 265 



than ten millimetres long. These cells ought to be prepared a 

 good while in advance, so as to be perfectly dry and hard when 

 they are wanted for use. Those of our readers who have a stove 

 can accelerate the drying by placing the cells near it for a few 

 hours. 



To Spread the Diatoms Uniformly over the Thin Glass. 



The Diatoms intended to be mounted dry ought to be held in 

 suspension in distilled water, absolutely pure ; that is to say, that 

 when evaporated on a thin glass, it shall leave no trace behind. 

 If they have been kept in eau-phe7iique or in alcohol (marine 

 gatherings), these liquids ought to be entirely got rid of. 



And here a little difficulty occurs which tact and practice will 

 soon surmount ; it refers to the quantity of Diatoms that ought to 

 be contained in two or three cubic centimetres of distilled water, 

 so that when once spread on the thin glass there shall be enough 

 without their being superposed one on the other, or allowed to 

 form into little masses, as frequently happens with the smaller 

 species. As a general rule, you may say that the water containing 

 the Diatoms in suspension should never have a milky appearance, 

 but only a slightly opaline tint, especially when the small species 

 are concerned. As to the medium-sized species and those still 

 larger, it is desirable to see them individually float in the liquid. 



The glass on which we are to spread the Diatoms ought to be 

 thin glass (English No. i), perfectly cleaned, for the least trace of 

 greasy matter will cause an immediate retreat of the liquid, and 

 the even spreading of the Diatoms will be rendered impossible. 



I always take the precaution, in order to avoid this difficulty, 

 to use new English glasses, which are cut mechanically and put up 

 in i-oz. and \-oz. boxes, and have not been handled. These can 

 be wiped between the finger and thumb with a bit of fine old 

 linen rag, while they are held by the edge with the other hand. 

 You may equally well use other glasses after they have been sub- 

 jected to the following treatment. 



The glasses are first placed in a lo per cent, solution of car- 

 bonate of soda, in which they are agitated for some seconds in 

 such a way as to bring all the surface into contact with the liquid. 

 This liqliid is poured off and twice replaced with distilled water ; 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. VI. s 



