208 HOW TO WORK 



or greenish tinge results from the action of iodine and sulphuric 

 acid. 



A strong solution of iodine may be obtained by employing a 

 solution of iodide of potassium to dissolve the iodine (one grain of 

 iodine and three grains of iodide of potassium, to one ounce of dis- 

 tilled water or glycerine). 



Schultz recommends the following iodine solution. Zinc is dis- 

 solved in hydrochloric acid ; the solution is permitted to evaporate 

 in contact with metallic zinc until it attains the thickness of a syrup ; 

 and the syrup is then saturated with iodide of potassium. The iodine 

 is next added, and the solution, if necessary, is diluted with water. 

 Professor Busk gives the following directions for preparing this 

 solution : one ounce of fused chloride of zinc is to be dissolved in 

 about half an ounce of water, and to the solution (which amounts 

 to about an ounce fluid measure), three grains of iodine, dissolved 

 with the aid of six grains of iodide of potassium, in the smallest 

 possible quantity of water, are to be added (Trans. Mic. Soc., 

 vol. I, p. 67). I have employed a solution prepared in this manner, 

 and can speak very highly of its utility. In making it, it is necessary 

 not to fuse the chloride of zinc much, or to use a very high tempera- 

 ture, as decomposition is very apt to take place. In testing starch 

 with this solution, it is advisable to add a very little water, as the 

 solution frequently will not act in its concentrated form. 



OF APPLYING TESTS TO MINUTE QUANTITIES OF MATTER. 



3O6. Method, of Applying Tests to Substances intended for 

 microscopical Examination. The matter to be tested may be placed 

 upon a glass slide, and, if necessary, a drop of water added, to 

 moisten or dissolve it, as the case may be. 



In these operations we usually require only a small drop of a 

 solution, and it will be found most convenient, in applying it to the 

 object, to take a drop from the bottle by dipping a stirring-rod into 

 it, and withdrawing it immediately. Enough will be found adhering 

 to the stirring-rod for the purpose required. The rod should not be 

 dipped in a second time, without being first well washed in water, 

 for if this be not scrupulously attended to, there is great danger of 

 conveying some of the substance intended for examination into the 

 test bottle, in which case the whole contents would be spoiled. 

 Without great care in all our manipulations, there will be much 

 danger of removing a portion of one substance from a glass slide 

 and carrying it to a specimen which is examined subsequently. 



