160 TEAXSACTIONS AND TKOCEEDINGS OF THE [Ses.s. lxih. 



converted to chloride. The completion of this reaction is 

 marked by the disappearance of the characteristic red 

 coloration of silver chromate. The surplus liquid is now 

 dried off by cigarette paper, and the excess of silver 

 chloride removed by sodium thiosulphate solution. After 

 thorough washing, the preparation is either mounted in 

 glycerine, or, after dehydration by absolute alcohol, in 

 Canada balsam. 



The staining is uniform throughout, and not merely 

 superficial. The colour is yellow, and the outline of the 

 mucilage is quite sharply marked off. 



In the chromatype process of photography, paper is 

 sensitised by treatment with a saturated solution of 

 potassium dichromate, mixed with either copper or nickel 

 sulphate. When dry, the paper is exposed in a camera. 

 A dark brown negative is thus obtained. It is now washed 

 over with solution of silver nitrate, and developed in 

 sunlight. Short development gives a negative, but further 

 development gives a positive. On being washed over 

 with sodium chloride solution, the image fades away 

 entirely, but reappears as a pale violet-coloured positive on 

 exposure to sunlight. 



The idea of applying this method to botanical microscopic 

 work was suggested by the fact that gelatine combines with 

 potassium dichromate in daylight, and with it forms an 

 insoluble compound. It appears that the mucilage of such 

 plants as Batrachospcrtnum, Rivularia, etc., can be impreg- 

 nated with dichromate also ; and when this is done, it is 

 then possible to impart a definite colour to the mucilage 

 by following out the chromatype method. 



Even if the dichromate did not combine with the 

 mucilage, the subsequent treatment with silver nitrate and 

 alkali chlorides would fix the colouring matter, and prevent 

 it being washed out. It has not been ascertained how 

 niucli dichromate is reduced by the action of light and 

 combined with the mucilage. It is possible to wash out 

 the most of it with water, but this should not be done in 

 practice if a deep coloration is required. 



Attempts to stain mucilage by steeping in potassium 

 iodide solution, sensitising with silver nitrate, exposing, 

 and developing, have not ]>ecn particularly successful. 



