242 



HARDWICKE'S SCIENCE-GOSSIP. 



world, about six hundred well-defined freshwater 

 species of diatoms. 



Their smallness. — Ehrenberg estimated that a cubic 

 inch could contain forty-one millions of Diatomaceoits 

 carapaces. I have found that one species averages 

 eight thousand in a cubic millimetre. Other precise 

 measurements show that a cubic millimetre can con- 

 tain twenty-seven million specimens of Navicula 

 pediculosa, and forty millions of Achnanthidium 

 delicatulum. These are our two smallest species. 



Their Endochrome. — Diatomine, or endochrome, 

 is the substance found within the silicious carapace, 

 or frustule. It is translucent, of an oily appearance, 

 strongly refracts the light, and its colour is brown, 

 tawny, or golden ; it corresponds to the chlorophyl 

 of other green Algae. The endochrome, under the 

 influence of heat, of alcohol, or acids, becomes a 

 beautiful emerald-green. It is thick and viscid, like 

 protoplasm, and its natural division in the frustule 

 takes place sometimes in the form of plates amongst 

 the placo-chromatic diatoms, and sometimes in the 

 form of granulations amongst the cocco-chromatic 

 diatoms. 



The endochrome is ordinarily motionless, very 

 rarely one sees it move under the form of granules 

 which appear to be gifted with a slow Brownian 

 movement. It contains a trace of iron, which is 

 found in the form of a peroxide when living diatoms 

 are calcined. It resists putrefaction for a long time. 

 The species which I ^collected in the Sahara, in 

 1873, an d preserved in the water in which I took 

 them, had still, four years after, their endochrome in 

 good condition. It remained translucent and yellow, 

 but its primitive form had changed and become con- 

 tracted. I have seen fossil diatoms, from a consider- 

 able deposit in Holland, and which, consequently, 

 had been buried for ages, show here and there 

 examples whose endochrome was still yellow and 

 transparent, although it had become thicker and 

 more plastic. Ehrenberg, in studying the " Kiesel- 

 giihr" of Hanover, noticed the same fact. I am 

 convinced that this only took place in those speci- 

 mens which had arrived at perfect maturity, and 

 whose two valves were still hermetically closed. 



Respiration. — Diatoms, like all the Algce, respire 

 by means of the carbonic acid gas which all waters 

 exposed to the atmosphere contain in a dissolved 

 state (gaseous nutrition). No carbonic acid, no 

 diatoms. They assimilate the carbon of the gas, 

 but the oxygen is set free and escapes, little by little, 

 under the form of minute bubbles. The carbon is 

 used in the formation and development of all the soft 

 part of the vegetable called the Thallus. At the 

 same time that they breathe it, they also take from 

 the water a portion of the mineral substances which 

 it contains in a dissolved state ; as iron, alumina, 

 lime, and, above all, much silica, which constitutes 

 its hard and transparent glassy frustule. 



If in a phial containing drinking-water and many 



living diatoms we inject a very slow current of 

 carbonic acid gas, and if the gas which escapes be 

 collected under the influence of light, experience 

 proves that the latter gas is richer in oxygen than 

 atmospheric air. 



Calcareo2ts Deposits due to Diatoms. — Nearly all 

 waters contain lime {Calcic carbonate). Lime, it is 

 true, is completely insoluble in water chemically 

 pure ; but when the water contains carbonic acid 

 gas, the gas renders the lime slightly soluble. In 

 proportion 'as the diatoms decompose this gas, the 

 dissolved lime is separated, and then it is either 

 precipitated or else it incrusts the mucilaginous 

 envelope, in the middle of which these Algae are 

 developed. It is especially the gelatinous spheres 

 where the Epithemia and some Synedra grow which 

 afford the microscope the prettiest groups of cal- 

 careous crystals. Where the water is quiet, the 

 lime that is set free settles to the bottom, and 

 partially forms the mud of stagnant waters ; but if 

 the water is running, the calcareous particles are 

 immediately swept on with the current. We must 

 not forget that in proportion as the carbonic acid 

 gas off the water is decomposed, the same water 

 dissolves a new amount of gas that it borrows from 

 the atmosphere, which gas serves in its turn to dis- 

 solve a new quantity of lime. These infinitely minute 

 plants maintain, then, in the water a constant move- 

 ment of mineral molecules and of the gas. This 

 role is incessant, and takes place in winter as well 

 as in summer, and the Count de Castracane was 

 right when he proved that the diatoms not only co- 

 operate directly by their silicious residuum, which 

 they leave after death, to form geological strata, but 

 also indirectly by the lime which they constantly set 

 free from the midst of the water. 



Their Silicious Frustule. — I do not believe that 

 there ever has been in nature a more marvellous 

 organic incrustation than the silicious envelope of 

 the diatoms. It is only with the most powerful 

 immersion lens, and giving a considerable linear 

 magnifying power (+ 1000 or 1500) that we are 

 enabled to resolve the finest striations of certain 

 species. But for the determination of species a linear 

 power of + 300 or 400 is nearly always sufficient, 

 and especially in employing oblique light. However, 

 all works treating on the microscope give the neces- 

 sary instructions. This silicious envelope resists 

 putrefaction for an indefinite length of time, and 

 remains intact at the bottom of the water as each 

 of the diatoms die, forming in this way in many 

 countries considerable deposits (Kieselguhr), and 

 which require thousands of years to form. This 

 silica resists acids, and even a dull red heat ; but 

 at an intense reddish-white heat it softens and 

 presents a half melted and vitreous mass. 



The following three analyses give the exact 

 chemical composition of the fixed frustule of the 

 diatoms. 



