ON AN EASY METHOD OF VIEWING DIATOMACEiE. 205 



bodies, which chiefly resemble the crystals of snow in their 

 hexagonal, stellate, arborescent shape and in the form of their 

 pinnae. If not, however, intimately allied, it is interesting 

 to observe and compare the manner of their change ; and a 

 continuation of these observations, varied by experiment 

 and the employment of other solutions, may yet afford 

 information on a subject which, as Mr. Spencer remarks, is 

 of peculiar interest, as uniting the confines of meteorology 

 and chemistry.* 



On an Easy Method of viewinfj certain of the Diatomace^. 

 By John Charles Hall, M.D., Physician to the Sheffield 

 Public Dispensary, &c, &c. 



We shall most certainly add not a little to the chances of 

 increasing our knowledge of the intimate structure of the 

 infusorial tribes, if any means can be suggested by which the 

 number of observers may be increased. Possessed of a good 

 microscope by one of our principal makers, a student may 

 imagine that he is in a condition at once to* ascertain the 

 exact form and nature of certain shells of Eacillaria, certainly 

 amongst the most difficult of test objects ; a very few trials 

 will, however, convince him that even the best-constructed 

 eighth will not fully display their peculiar markings without 

 some accessory instrument. For tliis purpose the achromatic 

 condenser of Mr. Gillet, as made by Mr. Ross, or the achro- 

 matic condenser of Powell and Lealand, or of Smith and 

 Beck, is usually employed. The purse, unfortunately, of the 

 most enthusiastic labourers is not always the heaviest ; a mi- 

 croscope is bought and added to from time to time, and the 

 condenser, so much coveted, must often be waited for some 

 years. 



We propose in the present paper to show how, for a few 



shillings, this apparatus may, for a time at least, be dispensed 



with ; and how the Pleurosigma Hippocamjms, Pleurosigma 



Jormosum, Pleurosigma angulatiim, and other individuals of 



this genus, may be shown in the most satisfactory manner ; 



* An infinite variety of crystalline forms, apparently allied to those 

 of snow and of camphor, is presented, when a drop of a weak solution 

 of common salt, to which a small quantity of tirea has been added, is 

 dried at a moderate temperature on a slip of glass held over a spirit-lamp. 

 The change produced by urea in the shape of the crystals of common salt, 

 from the octahedron to the dodecahedron, or some derivative, has been 

 long well known ; and the fact may be usefully employed, with certain 

 precautions, to determine, under the microscope, the presence of extremely 

 minute quantities of ?frea in animal and other fluids. — lEds.'] 



