Crystals for the Micro-Polariscope. 439 



It is not easy to lay down an nndeviating rule for the 

 strength of the solutions of the salts themselves. This must 

 be learned by experience, and will be found to require altera- 

 tion according to circumstances — the degree of solubility of . 

 the salt, for example, and the opacity of its crystals. Never- 

 theless, as some guide, I may mention that in tentative experi- 

 ments I always begin with a saturated solution — that is, one 

 which contains as much of the salt as water at the ordinary 

 temperature will dissolve. If I find that this gives too thick a 

 layer of crystals, I dilute the solution with water, but it will be 

 found in most cases that considerable latitude can be allowed 

 in this respect without materially affecting the result. The 

 solutions must be filtered just before use, and the access of dust 

 into them or into the capsule carefully prevented. 



We will suppose now that we are about to make some slides 

 of chlorate of potash. The best strength for this solution is 

 that which I have just mentioned, viz., one which is saturated 

 at 60° Fahr. The first step in the process is to clean the slide, 

 or rather slides, for a dozen or more may be covered with 

 crystals with the expenditure of but little more trouble than is 

 required for one. Holding the glass slip by its short edges 

 between the finger and thumb, both surfaces are well rubbed 

 over with the india-rubber pad, the latter having been pre- 

 viously dipped into the caustic alkaline solution. When several 

 slides have been thus consecutively treated, the first is held 

 under a stream of water from a tap until the water flows freely 

 over it. If the least appearance of greasiness is perceptible, 

 the treatment with the caustic alkali must be repeated until 

 the glass is perfectly clean. While this part of the operation 

 is being conducted, the solution of chlorate of potash may be 

 filtering into the capsule. This filtration at the last moment 

 is quite a necessary precaution, in order to ensure against the 

 presence of any floating particles of foreign matter. The 

 liquid is next heated to its boiling point, and a clean slide 

 carefully rinsed with distilled water and gently dropped into 

 it } another slide is then rinsed and immersed, and so on until 

 four or five, or even more, have been similarly treated. The 

 lamp is then removed, and each glass slip carefully taken out 

 by its edges with the forceps, and set up on end on a bit of 

 blotting-paper to drain. In the case supposed — that of chlo- 

 rate of potash — both surfaces of the slides will immediately be 

 covered with a beautifully regular and even layer of small 

 crystals, and if the cleaning process have been carefully con- 

 ducted, it will be found that either side may be used fox 

 mounting. The slides are now — unless the salt employed be 

 liable to alteration by exposure to the air — put away in a 

 grooved box until it is desired to finish them off. 



