168 TIOW TO Vs'ORK 



microscope, made by Mr. Highley, and others which preceded it, 

 have been recommended for the same purpose, but to these there is 

 the objection that only one very small spot can be examined, unless 

 very low powers be employed, and it is of course a mere chance if an 

 object happens to fall in the centre. It seems to me that every 

 instrument, however simple, should permit the object to be moved 

 about, at least a three-eighths of an inch in every direction. The 

 difficulties of effecting this are not great. Seep. 15. On collect- 

 ing diatoms, see p. 147. 



The siliceous remains of the diatomaceae may be separated from 

 guano and other deposits as follows : The organic matter and carbo- 

 nate and phosphate may be removed by boiling in nitric acid, and 

 the remaining deposit diffused through water and collected as before 

 described, but I much prefer to destroy the organic matter by burn- 

 ing the deposit in a platinum basin, and allowing it to remain for 

 some hours at a red heat until the black carbonaceous matter has 

 burnt off, leaving a pure white ash. The phosphates and carbonates 

 may be removed with dilute nitric acid, and the deposit washed. In 

 this way the shells are not so liable to be broken as they are when 

 the deposit is boiled for some time in strong acid. 



Siliceous shells of certain diatoms are represented in pi. XL, 

 figs. 251, 252, 255. There is much difference of opinion as to the 

 cause of the markings in many of these. Mr. Hunt considers the 

 dots on pleurosigma represented in fig. 252, as elevations not 

 depressions (Mic. Journ. vol. Ill, p. 175). 



The skeletons of diatoms in Bermuda earth and other deposits of 

 a like kind may be obtained by boiling the powder for a short time 

 in a weak solution of potash and then washing in successive portions 

 of distilled water according to the plan described in page 85. 



For the details of mounting diatoms, the reader is referred to 

 Mr. Davies' little work on " Mounting Microscopical Specimens. 



ON THE MOVEMENTS OF LIVING BEINGS AND UPON PURELY VITAL 



MOVEMENTS. 



I shall now offer a few general observations upon the nature of 

 the different kinds of movements which occur in living things, many 

 of which have been referred to in the foregoing sections. 



Hitherto many of the movements occurring in living things have 

 been referred to the property of contractility, and strange to say, the 

 very authorities who never lose an opportunity of condemning those 

 who attribute any changes in things living to the influence of a 

 peculiar force or power vitality, consider that movements are 



