126 



SCIENCE-GOSSIP. 



[June 1, 1866. 



In the preparation of the first two, I nse satu- 

 rated solutions, made with cold distilled water, and 

 subsequently strained through bibulous paper, to 

 rid them of the mechanical impurities generally 

 found in them. I must here remark that these 

 crystals, and many others, may be made to assume 

 two forms, which, to the unpractised eye, are totally 

 dissimilar. To prove this, place a drop of the solu- 

 tion of either of these salts upon a glass slide, 

 previously made perfectly free from grease. Let it 

 evaporate, and the result will be crystals, which 

 may be compared to the fronds of ferns in outline. 

 Now place another drop upon a similarly cleaned 

 slide, evaporate it, but not too quickly, over a hot- 

 water bath, gently stirring it with a thin glass rod. 

 In this case the crystals will be found to exhibit 

 rhombic prisms, separate and distinct, which give 

 varied and beautiful colours under the polariscope, 

 even without the selenite, and showing up well upon 

 the dark ground. Of the two forms, the latter 

 seems preferable, not only on account of their 

 greater beauty, but also because they exhibit more 

 clearly the true form of the crystals, with reference 

 to the "system" in which they are placed by 

 crystallographers. The medium used for mounting 

 may be either Canada balsam or castor-oil. No 

 special directions for the use of the former are 

 required; all that is necessary being to see that 

 sufficient water be driven out from the crystals, or 

 water bubbles will be formed when mounted, which 

 are extremely difficult to get rid of. On the other 

 hand, care must be taken that the water of crystal- 

 lization be not parted with, by too long an exposure 

 to the action of the hot-water bath, in which case 

 the crystals will present only an amorphous mass, 

 of little beauty, and still less use. Should castor- 

 oil be the medium employed, only just sufficient to 

 fill the space underneath the thin glass cover must 

 be used, since the removal of the superfluous oil is 

 attended with much trouble. The exact quantity 

 varies with circumstances, and must be learned by 

 experience. The method I have adopted to effect 

 this, is to put a weak clip with a cork disk under- 

 neath, upon the mounting, >and, after scraping away 

 the crystals around the cover with a penknife, to 

 soak up the expressed oil with blotting-paper. 

 When this method of cleaning has been carried as 

 far as possible, I apply a saponifying compound to 

 the remains of the oil (taking care that it does not 

 run in under the cover), which renders the adhesion 

 of the cement used to fasten down the cover more 

 certain. One side is then secured with gold size, 

 and sufficient time given it to harden thoroughly; 

 then another side is treated in the same manner, and 

 so on, allowing ample opportunity for each side to 

 dry perfectly before the next is proceeded with. 

 When all are done, the clip may be removed entirely, 

 but until then it should only be shifted as occasion 

 may require. Two or more coats, either of gold-size 



or asphalt, must be added, observing to give each of 

 them plenty of time to harden before the next is 

 applied, more especially -when asphalt is used over 

 the gold-size. This completed, the slide may be 

 safely placed in the cabinet. 



The foregoing remarks apply equally to the sul- 

 phate of magnesia, or Epsom salts, better known 

 under the latter name. Castor-oil is preferable for 

 this salt, as balsam gives some trouble, owing to a 

 cause unknown to me. 



With the chlorate of potash there is no trouble, 

 unless the crystals are wanted separate, and not 

 agglomerate. I have succeeded in obtaining the 

 latter form in two ways; in both cases using a 

 saturated solution in hot water. The first plan is to 

 tilt the slide at rather an acute angle with the per- 

 pendicular, and to let drop a good quantity of the 

 solution over the centre of the slide, in such a 

 manner that the superfluous liquid runs off immedi- 

 ately. The contact of the hot solution with the 

 cold glass instantaneously develops the crystals 

 (which have a square outline) in a separate form. 

 The second method is to keep the slide horizontal, 

 and in its centre to place a drop of lukewarm dis- 

 tilled water, into which the hot solution is dropped 

 from a pipette. This gives results similar to those 

 produced by the first plan ; but the water must be 

 drained off, and the slide dried by gentle heat. 

 When quite dry, mount in balsam in the usual 

 manner. The disadvantage of this last plan is, that 

 the crystals frequently seem to be broken at the 

 angles, and do not exhibit such fine colours as in 

 the first case. Occasionally, however, very large 

 and beautiful crystals result, which it is well worth 

 the cost of a few failures to obtain. 



Tartaric acid is one of the easiest salts to prepare. 

 I prefer to evaporate this rather quickly over a 

 lamp, at a temperature pleasantly warm to the hand. 

 By holding the slide, which should be slightly tilted, 

 in the fingers, too great a heat will be readily pre 

 vented. Mount in balsam, which is best, or in 

 castor-oil. 



Citric acid offers some peculiarities. When first 

 I tried this, I used a saturated solution, which 

 I heated on the glass slide, until I was tired, 

 without result further than that the drop of the 

 solution had assumed a viscous, glass-like appear- 

 ance. The slide was put away until the following 

 morning, when it was found almost covered with 

 circular expansions, forming a fine object, save that 

 its beauty was marred by imperfect forms which 

 appeared between the crystals, quite spoiling it 

 Avhen viewed with the dark ground obtained by the 

 polariscope. These imperfect forms can only be got 

 rid of (so far as my experience goes) by keeping the 

 slide upon the hot-water bath until the crystals are 

 fully formed. This, however, is attained at the cost 

 of fissures in the crystal itself, unless it be carefully 

 watched, and removed from the bath directly they 



