WITH THE MICROSCOPE. 89 



firm jelly on cooling. If gelatine is employed it must be soaked for 

 some hours in cold water before it is warmed. About an ounce of 

 gelatine to a pint of water will be sufficiently strong, but in very hot 

 weather it is necessary to add a little more gelatine. It must 

 be soaked in part of the cold water until it swells up and becomes 

 soft, when the rest of the water, made hot, is to be added. Good 

 gelatine for injecting purposes may be obtained for two shillings a 

 pound. 



i8. Colouring Matters. The colouring matters usually employed 

 in making opaque injections are the following -. Vermilion, Chromatc 

 of Lead, and White Lead. Of these, vermilion affords the most 

 beautiful preparations, but chromate of lead properly prepared is 

 much cheaper, and it may be obtained in a state of more minute 

 division. White lead forms a good colouring nutter, but its density, 

 and its tendency to become brown and black when exposed to the 

 action of sulphuretted hydrogen, formed in the decomposition of 

 the tissues, are objections to its use. 



169. Vermilion of sufficiently good quality can be purchased of 

 all artists' colourmen for six or eight shillings a-pound. If upon micro- 

 scopical examination a number of very coarse particles be found in 

 the vermilion, it will be necessary to separate these by washing in 

 water in the manner described in 160. 



HO. The Chromate of Lead is prepared by mixing cold saturated 

 solutions of acetate of lead and bichromate of potash. The yellow 

 precipitate is allowed to settle, and after pouring off the clear solution 

 of acetate of potash resulting from the decomposition, it is shaken up 

 with water, again permitted to settle and mixed with strong size or 

 gelatine. After being strained through muslin the mixture may be 

 injected into the vessels. 



171. The Carbonate of Lead or White Lead is prepared by 

 mixing saturated solutions of acetate of lead and carbonate of 



O 



soda. The precipitate is to be treated as the former one and mixed 

 with size. 



In preparing opaque injections, the observer should bear in mind 

 that the colouring matter should be well mixed with the size, otherwise 

 the vessels will not be uniformly filled, and it is better to employ a 

 small syringe rather than a large one, as there is not so much chance 

 of the colouring matter separating from the size before the mixture is 

 forced into the vessels. In all cases the mixture may be made in a 

 mortar, poured into one of the injection cans, fig. 151 a, and strained 

 into another through muslin just before it is injected into the vessels 

 of the animal. 



