CHEMICAL RE- AGENTS. 131 



surface of the preparation. The colour is more intense 

 where the mixture is more concentrated; it frequently 

 happens that many spots remain uncoloured. The colour 

 changes after some time, the blue being frequently changed 

 into red after twenty-four hours. 



7. A Solution of Chloride of Zinc, Iodine, and Iodide of 

 Potassium. A drop of this compound solution, added to 

 a preparation placed in a little water, produces the same 

 colour as iodine and sulphuric acid. This solution, which 

 was first proposed and employed by Professor Schultz, is 

 more convenient in its application than iodine and sulphuric 

 acid, and performs nearly the same services, while it does 

 not, like the sulphuric acid, destroy the tissues to which it 

 is applied. It is prepared by dissolving zinc in hydro- 

 chloric acid; the solution is then saturated with iodide of 

 potassium : more iodine is to be added if necessary, and 

 the solution diluted with water. 



8. Nitric Acid, or what is better, chlorate of potass and 

 nitric acid, as an agent to effect the isolation of cells. The 

 mode of employing this agent, also discovered by Professor 

 Schultz, is as follows : 



The object, a thin section of wood, for instance, is intro- 

 duced, with an equal bulk of chloride, or chlorate of potass, 

 into a long and moderately wide tube, and as much nitric 

 acid as will at least cover the whole. 



The tube is then warmed over a spirit-lamp ; a copious 

 evohition of gas takes place, upon which the tube is re- 

 moved from the flame, and the action of the oxidising agent 

 allowed to continue for two or three minutes. The con- 

 tents of the tube are then poured into a watch glass with 

 water, from which the slightly cohering particles are col- 

 lected and placed in a tube, and again boiled in alcohol as 

 long as any colour is communicated. 



They are again boiled in a little water. The cells may 

 now be isolated under the simple microscope by means of 

 needles. The boiling with nitric acid and chlorate of 

 potass should never be carried on in the same room with 

 the microscope, the glasses of which may suffer injury 

 from the vapours. The same remark applies to all chemical 

 vapours. 



Thin sections of vegetable tissue are warmed for half a 

 K 2 



