88 TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



solved the question in a convincing manner by sections of various 

 diatoms. Messrs. Prinz and Van Ermengem have based their con- 

 clusions on sections of diatoms found in the Cemenstein of Jutland. 

 Mr. Haughton Gill has had recourse to a method as ingenious as it 

 is novel : by employing a number of chemical solutions he has, by double 

 decomposition, produced in the very alveoles themselves various deposits 

 (principally of the metallic sulphides) and has at the same time clearly 

 proved the existence of cavities in the valve. Mr. Gill has thus treated 

 the most varied genera, those with the finest as well as the coarsest 

 structures, and all have given the same results. 



The first experiments of Mr. C. Haughton Gill were made by soaking 

 the valves with ferric chloride, then by treating them with a solution 

 of potassium ferro-cyanide, thus producing deposits of Prussian blue, 

 which are fixed in the interior of the alveoles, whilst the exteriors of 

 the valves can be cleansed by washing. Later he used the double 

 chloride of sodium and platinum decomposed by the subsequent action 

 of oxalic acid ; then mercury nitrate with ammonium sulphide, giving a 

 black deposit of mercury sulphide, and then finally silver nitrate and 

 ammonium sulphide, which is suitable for diatoms with very fine 

 alveoles such as Pleurosigma angulatum, etc. 



Again quite recently Mr. Albert Brun, of Geneva, the son of the 

 celebrated and learned diatomist, confirmed in his turn that the before 

 mentioned diatom-beads are alveoles or depressions (') by a very 

 ingenious method, which consists in examining the bodies under the 

 microscope in liquids of various indices. 



When a particle of a transparent body, submerged in a liquid, is 

 microscopically examined and exactly focussed and the tube of the 

 microscope is then raised, the object will appear to have a brilliant centre 

 if its index of refraction is greater than that of the liquid, and to 

 have a dark centre if its index is smaller than that of the liquid. 

 By lowering the tube the exactly opposite phenomena will be produced. 

 Should the object occupy an appreciable portion of the field, a brilliant 

 band is seen moving across the centre of the fragment, and the whole 

 object to be lightened up whilst the tube is being raised or else a 

 brilliant band moves away towards the circumference, and the object 

 becomes darkened according as its index is greater or smaller than that 

 of the liquid. By lowering the tube the phenomenon is inverted. By 



( ' ) Proctfde' de determination de l'indice de retraction de cristaux ou fragments de cristaux 

 extremement petits, par A. Brun. Archives des Sciences Physiques et Naturelles de Geneve, 

 August, 1894. 



