114 A HALF-HOUR WITH 



glass cover, set aside in a cool place to dry ; this 

 will usually take about half an hour. If the 

 experiment has been successful, the crystals will 

 appear like fern fronds. (See fig. 4, plate 9.) 



This figure will give some idea of the elegance of 

 form and beauty of colour ; but it is beyond the 

 skill of any artist to do justice to the beauty of 

 a good slide. The sulphates of nickel and iron are 

 also very good when crystallized out of gelatine. 



With chlorate of potash, a totally different form 

 of crystallization is produced, the crystals being 

 tabular and large. A very remarkable effect is 

 produced when a small quantity of a solution 

 of barium is added ; the barium will be found 

 to have crystallized in small moss-like tufts at 

 the angles. Chloride of barium mixed with 

 the gelatine solution assumes a dendritic form, 

 somewhat resembling sulphate of copper, but 

 polarizes differently. Gum arabic may be substi- 

 tuted for gelatine ; the modus operandi is, how- 

 ever, similar ; albumen (white of egg) requires to 

 be dried before it is added to the distilled water, 

 which must be only slightly warmed. The stu- 

 dent cannot do better than try the effect of the 

 different media ; some salts ' do better with gela- 

 tine, others with gum ; for example, he will be 

 able to produce more effective slides of tartrate of 

 soda with gum than gelatine. 



Platino-cyanide of Magnesium must be prepared 

 without heat, as warmth alters the colour of the 

 crystals. We have obtained the best results by 

 adding a few crystals to a drop of the gelatine solu- 

 tion previously placed on the slide, stirring them 

 with a stout bristle until dissolved, and then allow- 

 ing them to slowly recrystallize. These crystals, 

 like the iodo-sulphate of Quinine, will analyze a 

 polarized ray. They are best mounted in dammar. 



