ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1099 



normal solution of 40 gr. The refractive index in this case becomes 

 nearly 2. 



The second medium is realgar, the transparent sulphide of arsenic 

 dissolved in bromide of arsenic by aid of heat. Both of these sub- 

 stances should be pure and the mount should be boiled as long as 

 bubbles are readily given off with considerable heat, and when cold 

 the cover should be more firmly attached than with balsam. These 

 mounts are of a deep lemon-yellow colour, and the compound has a 

 refractive index of 2 • 4. 



Excellent and even better mounts, as to permanence, may be 

 made by using realgar only by sublimation. A bit of the realgar is 

 put on a plate of mica about 1 in. square, and thick as a penny. 

 This is melted by strong heat of a spirit-lamp. On this mica plato 

 is placed another, with a hole 5/8 in. in diameter, and above this a 

 thin glass plate with a hole slightly 

 less than the glass cover on which the Fig. 200. 



diatoms are mounted. In fig. 260 a ^ 



and 6 are the two mica plates, c the ^ff5^ 



E?^ 



glass plate, and d the cover, with the ^^^^^^^^^ 

 diatoms facing the realgar. The whole 



is now supported on a metal ring. A strong heat will volatilize the 

 realgar without change, and often a clear deposit is made all over 

 the diatoms and under side of the cover, and the latter can now be 

 mounted in balsam ; but if bubbles are formed in the operation, as 

 probably will be the case, the heat must be continued till these 

 disappear and, as the deposit will now be thickest at the centre 

 just over the realgar, the mount may be finished by putting the 

 cover, realgar side down, on a clean slide and on the top of it to 

 prevent breaking, a piece of thick glass, and then, grasping tightly 

 with forceps to give pressure, heating strongly over a spirit-lamp. 

 The realgar will soften (it must not be melted else bubbles will form 

 which cannot be removed) and spread out, more or less, between the 

 cover and slide making a nice clear mount. The colour of the heated 

 realgar is very much deeper than when cold. Instead of the solid 

 realgar a drop of the solution in bromide of arsenic may be used ; 

 but in this case it must be boiled to expel the most of the bromide, 

 bcfjre the cover is placed above it ; the solid compound now melts at 

 a much lower temperature than the realgar alone. These mounts 

 will not change, but those made from the solution directly will, if 

 the irigrodiouts are not entirely pure, containing no excess of either 

 sulphur or arsenic. As bromide of arsenic will dissolve both sulphur 

 and arsenic there is always danger, if the realgar is not pure, that 

 there will be an excess of one of these, and if so tlie mount will either 

 crystallize or granulate. 



Prof. Smith also writes* that he is now testing still another medium 

 of somewhat higher index than the stannous chloride, a full account 

 of which will appear in due time. 



Smith's New Cement.t ~Pr<;f. IT. L. Smith has comrauuicatcd 

 the rcsultH of some recent experiments he has made with a new cement, 

 ♦ Amtr, Mon. Mic-r. Journ., vi. (1885) p. 182. f I'^id- 



