350 On the Manufacture of Crystals of Sulphate oflodo-quinine. 



It is essential in iodizing the plate that the exposure to the 

 vapour be not too long continued ; the time necessary will of 

 course depend on the temperature of the apartment, about three 

 hours at 50° F. being generally necessary. The reason of this 

 precaution will be at once evident upon making the experiment, for 

 the crystals assume a rich golden yellow colour, both by reflected 

 and transmitted light ; the field will therefore, when the two 

 plates are parallel, be intensely yellow, a most objectionable 

 colour for the examination of objects. The crystals have lost 

 the power of- stopping the yellow rays, and the complementary 

 relation of the body to the superficial colours appears to be lost 

 also — a very remarkable fact. 



The rationale of the periodizing process appears to be the 

 addition of as much iodine to the crystal as will be sufficient to 

 communicate the exact complement of yellow to neutralize the 

 red and blue rays of the purple " body-colour." These rays are 

 now absorbed by the plates as they would be by yellow glass. 

 If too much iodine be added, an intense yellow light becomes 

 transmissible when the crystals are parallel, and the plate becomes 

 rotten and brittle, and will be almost certainly destroyed in 

 mounting, even if it be exposed to the air for some time before 

 attempting to do so, by which process the superadded iodine 

 again volatilizes, clearly showing that no chemical union could 

 have existed. 



Having so far prepared the " artificial tourmalines," it merely 

 remains to cover them by another plate of thin microscopic glass, 

 interposing some highly refractive cement or varnish between 

 the two plates. 



Several cements offer themselves to our notice, but some 

 selection is necessary. Canada balsam is one of the best ; how- 

 ever, in using this it is necessary to have it very fluid, and 

 not to employ much heat in the process ; in fact, I believe it 

 best to use it so as to be fluid at the ordinary temperature. I 

 have found, however, that it appears to attack the crystals and 

 dissolve out the iodine ; in order to correct this destructive ten- 

 dency, it is essential to saturate the fluid Canada with iodine at 

 the ordinary temperature. This is best done by warming some 

 small quantity of the balsam in a test-tube or thin bottle and 

 dropping into it some crystals of iodine, agitating them well 

 together by a glass rod ; giving time to cool and the excess of 

 iodine to subside, it is fit for use. It is merely necessary to take 

 a small drop of this fluid on the end of a glass rod, place it on 

 the larger and clean glass circular disc, then invert the disc, 

 carrying the crystal, upon it, press the two together gently and 

 steadily with the finger or a glass rod, or piece of stick, taking 

 great care not to use much force, in case the circles or crystal may 

 sustain injury. Now remove all the extra Canada balsam from 



