54 



THE AMERICAN MONTHLY 



[March, 



described above witli great satis- 

 faction. 



The iinest rotifers will be found 

 among or attached to submerged 

 plants in sheltered localities. 



At this season there may be 

 found many kinds of rotifers and 

 vorticellas, water-fleas, water-beetles, 

 hydras, and many other beautiful 

 and interesting creatures. 



Some "Expert" Evidence. 



In the course of the recent trial 

 of the Rev. Mr. Hayden, in Con- 

 necticut, a large number of scien- 

 tific gentlemen were called upon for 

 their testimony. Some of the evi- 

 dence that was thus brought to bear 

 is of an interesting character, not 

 only as an indication of the value of 

 the microscope in such cases, but 

 also as showing to what extent we 

 can rely upon the results of the ex- 

 periments of individuals. 



We will briefly review the testi- 

 mony that was based upon micros- 

 copical examinations. 



The microscope was employed 

 for two purposes on this trial ; for 

 the examination of specimens of 

 white arsenic, and for the detection 

 of blood-coi'puscles. 



In the study of arsenic-crystals 

 and the various kinds of arsenic in 

 the market, Professor E. S. Dana 

 had spent several months, and his 

 testimony was of a satisfactory na- 

 ture throughout. Samples of arsenic 

 from different manufactories vary 

 in character more or less, and this is 

 also true of the products from the 

 same factory at different times. The 

 peculiarities are to be found in the 

 proportionate number of crystals, 

 amorphous particles, and dust, in the 

 size of the crystals and masses, and 

 in the general appearance of the 

 specimens, particularly when exami- 

 ned by reflected light, which shows 

 the crystal-faces either dull or 



polished, or, when they have been 

 subjected to the slow action of 

 solvents, striated and marked. 



Among the causes whicli prevent 

 uniformity in the products of the 

 same factory at different times, may 

 be mentioned the rapidity of the 

 crystallization — which varies with 

 the temperature — the larger crystals 

 forming most slowly ; the density of 

 the vapor in the condensing cham- 

 bers ; and the presence or absence of 

 foreign matters. 



Although it is admitted that the 

 product of a factory varies from 

 time to time, it appears that it is 

 possible to distinguish the arsenic of 

 different makers with some degree 

 of certainty, and Professor Dana 

 believes that he could certainly 

 identify the brands in his possession 

 known as "• Dayton, " " Garland," 

 " Welsh," and " Dragon," by micro- 

 scopical examination. 



Many specimens of arsenic were 

 examined by Professor Dana, but 

 we will only refer to a few of them 

 in illustration of the character of 

 his observations. The arsenic from 

 the Tavestock works which he 

 visited, contains never less than one- 

 half his bulk of minute, but perfect, 

 octohedral crystals. The " Garland " 

 or glass-arsenic contains no crystals, 

 hence, when ground it appears as an 

 amorphous powder. By examining 

 fourteen slides of a certain sample, 

 one-third of it was found to consist 

 of distinct crystals, most of them 

 varying in size from ttVo to ytoo 

 of an inch, taking the maximum 

 and minimum diameters. By re- 

 flected light the crystals showed an 

 imperfect lustre. 



Another sample was made up al- 

 most exclusively of regular octohe- 

 dra, which formed nine-tenths, never 

 less than three-fourths, of the entire 

 mass, in size about toW ~ Woro"» 

 some as large as -^^" - -^fo"? with 

 smooth, and brilliant surfaces. 



