506 SUMMARY OF CURRENT RESEARCHES RELATING TO 



some black specks of foreign matter . . . and I now believe that they 

 are nothing else than aerial motes attracted to the sulphur ... I distil 

 the sulphur in a small retort at such a rate that it condenses in the beak 

 and flows down into a porcelain basin put to receive it. After a few 

 hours, or nest day, the cake in the basin is detached by gently warming 

 the basin until the sulphur in immediate contact with it melts, and 

 displacing the cake or turning it over. When cool again the cake is 

 broken up with as little friction as possible and put into a bottle. Dust 

 and even small fragments should be rejected. The distillation is 

 repeated in exactly the same way for the sake of being as sure as 

 possible of purity, though I have no direct evidence that the second 

 distillation is necessary if the sulphur started with is fairly pure. If 

 "pure" sulphur cannot be obtained, roll sulphur is preferable to 

 flowers. 



"The arsenic is also distilled twice, starting with the commercial 

 substance. The second distillation should leave no residue whatever. 

 A hard glass tube is appropriate for this distillation, as it requires a' red 

 heat. The tube is broken to get the arsenic out, and only the clear 

 shining black crystalline parts are preserved. 



" We must first make a definite preparation, and then use it so that 

 it remains of the same composition throughout. The sulphur and the 

 arsenic, purified as already described, are weighed out in proper pro- 

 portions with as little manipulation as possible; and the quanity taken 

 may be such as will extend about two-thirds up a test tube 3 in. long 

 and | in. in diameter, when the materials are in as large lumps as possible 

 for the weighing and putting into 1 such a tube. (The total weight of 

 materials taken may conveniently be from 30 to 40 grains, or 2 to 

 2| grin.) The tube is then gently heated in the flame of a Bunsen 

 burner till the sulphur melts and runs down to the bottom, carrying the 

 arsenic with it. The lower part of the tube is then heated rather more 

 strongly until an internal glow and commotion indicate that combination 

 is taking place. Just at this time it is well to withdraw the tube from 

 the flame to moderate the action. In a few seconds heat is again applied 

 and continued until the material, which is now throughly fused, boils. 

 It may be boiled up two or three times to ensure uniformity, and then 

 put away to cool. After some time, preferably an hour or two or more, 

 the tube is broken away from the little lump of material, the bits of 

 glass are carefully removed from it, and the brittle preparation may be 

 cut up by the large blade of an ordinary pocket knife into suitably sized 

 pieces for preservation or for use. For the reason already given it 

 should be handled and rubbed as little as possible. It should be very 

 bright and of a rich amber colour. Any suggestion of brownness 

 indicates impurity. 



" Mounting the Diatoms. — The covers and the slips must be thoroughly 

 clean. The best method that I know of is to put a drop of glacial acid 

 in the middle of a slip, and to put another slip upon it so that the drop 

 spreads over the central portion of both. As many as will be required 

 are so treated, and then eacii pain's separated, well rinsed under the tap, 

 and wiped and polished with an old soft linen pocket handkerchief, and 

 the specially cleaned sides of the pair may be put together a^ain to keep 



