52 CULTIVATION OF DIATOMS. 



The purity of the products employed in the solution is, according 

 to Mr. Gill, essential. If one wishes to succeed, too much care cannot 

 be taken to ensure the absence of every trace of arsenic, mercury or 

 silver. The presence of arsenic which is so often found in ferric chloride, 

 in consequence of its being frequently manufactured with impure hydro- 

 chloric acid, is one of the commonest causes of failure. 



As Dr. Miquel also points out, Mr. Gill discovered that the direct 

 rays of sunlight are absolutely harmful to the greater part of the cultivation. 

 He exposed his flasks to the direct sunlight on a board close to some 

 glass windows which were situated N.N.W., at the same time inter- 

 posing between the glass and the flasks a plate of pale green glass of the 

 height of the flask and a wooden board slightly higher than the liquid. 

 All the cultivations had been planted with one or more frustules, 

 transferred with a capillary tube. "This," said he, "is the simplest 

 and quickest method and also ensures absolute certainty that there is 

 no other diatom in the final drop which is to be used for the purpose 

 of planting the flask. 



The experiments of Mr. Gill were tried on a sufficiently large number 

 of forms, among which we may mention PL Angulatum, Cymatopkura 

 solea and Ellipiica, various Nitzschia, Cymbella and Navicula. All these 

 forms were made the subject of numerous successive cultivations, and he 

 had about ioo in his possession at the time of his decease. Some of 

 these cultivations are unfortunately dead, perhaps from want of assimilable 

 matter or for other unknown causes ; but a considerable number of them 

 are still living. All of these, with voluminous notes by Mr. Gill, are 

 in my possession. I hope to be able to follow these cultivations 

 through a number of generations, and if the opportunity occur I intend 

 to publish the results obtained from them. 



E. Results of Experiments with Cultivations. 



Dr. Miquel has published in " Le Diatomiste " the technical methods of 

 treating cultivations, and his experimental researches in the physiology, mor- 

 phology, and the teratology of diatoms, which have resulted from the before- 

 mentioned cultivations, have been published in the " Annales de Micrographie." 



We shall in a few words summarise the results obtained by the 

 learned observer, but we refer the reader who desires a detailed account 

 to the above-mentioned publication in which the experiments are fully and 

 minutely described. 



i. Effect of Temperature. A. Damp heat. Diatoms flourish at a 

 temperature between 15 and 30 C. 



