ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 763 



a concentrated solution of potash or soda. This should be followed 

 by a washing of the tissue in distilled water. When dry, the 

 fibres are separated ; a third part is submitted to the action of the 

 reagents. 



We must here confine ourselves to pointing out the distinction 

 which iodine and sulphuric acid, or chloriodide of zinc, allow us to 

 establish between the elements which they colour blue and those 

 which they colour yellow. To establish this distinction, Vetillart 

 advises the use of a solution of iodine freshly prepared by saturating 

 with this metalloid 100 gr. of distilled water, to which has been pre- 

 viously added 1 gr. of iodide of potassium. For the sulphuric acid 

 he recommends 2 volumes of concentrated glycerin to be added to 

 1 volume of distilled water, into which solution is to be introduced 

 little by little 3 volumes of commercial sulphuric acid marking 66° 

 Baume. The vessel in which this operation is carried out should be 

 surrounded by water. 



The fibres which are subjected to the action of tLese reagents 

 should be as dry as possible ; with this view they are exposed to heat. 

 The fibres are placed on the glass slide, and one or two drops of the 

 iodized solution are added. When the fibres are vrell soaked, the 

 excess of liquid is removed by filtering-paper. The cover-glass is 

 then placed over the fibres, and a current of the solution of sulphuric 

 acid is made to pass beneath the cover-glass. The reactions produced 

 are then observed. Amongst Dicotyledons, jute is coloured yellow ; 

 flax, hemp, sunn, and cotton are coloured blue ; amongst Monoco- 

 tyledons, Phormium tenax and Agave americana turn yellow ; alfa and 

 esparto completely blue. Those who are interested in this micro- 

 chemical examination of textile fabrics may consult with advantage 

 the work of Vetillart, which is full of details for which there is no 

 room here. 



VII. Preservation. 



The processes for preserving histological preparations being gene- 

 rally well known, there need be but little said on the subject. 



Glycerin is the liquid most often used for this purpose. There 

 are, however, many cases in which it is not generally known that it 

 is worthless. It must not be used for the Florideae, diatoms, or 

 bacteria. The cell- walls of the Floride^, especially when they 

 have not been previously immersed in absolute alcohol, swell up in 

 glycerin to such an extent that the form of the cells is no longer 

 recognizable. The markings on the diatoms are not shown clearly, 

 and the cell-walls of the bacteria become so transparent in glycerin 

 that it is very difficult to see them. 



These algge, on the contrary, keep very well in glycerin jelly. 

 Nordstett * especially recommends it for the Desmidiese ; he prepares 

 it by mixing hot gelatin (pure), 1 part ,• distilled water, 3 parts ; 

 glycerin, 4 parts ; which he afterwards decants. 



* " Om anwandandet af gelatin-glycerine vid unterscikning og preparering af 

 Desmidieer," But. Notiser, 1«76, No. 2, 



