CULTIVATION OF DIATOMS. 53 



At 35 C. many species, especially the large forms, are considerably 

 affected ; after having been subjected to this temperature for two hours, free 

 diatoms no longer move, and in the greater part of them the endochrome 

 can be seen contracted, and showing signs of a large number of air- 

 globules. At the end of 15 days to 2 months most species, if 

 planted in a new maceration, are restored to life. 



At 38 many species are absolutely destroyed. 



At 40 life becomes extinct in the greater part of the diatoms 

 belonging to our temperate climate. After being submitted to this tempera- 

 ture for two hours the protoplasm becomes perfectly glanular. At the 

 end however of a few days Navicula, Mclosira, Fragilaria, as well as 

 Surirella ovata, and Nitzschia thermalis are restored to life, while large 

 Synedra, Nitzschia, &c, are absolutely destroyed. 



Between 40 and 45 all species are irrevocably destroyed. 



Observations show that, in proportion as the temperature increases, the 

 endochrome contracts from the walls of the valves and throws off a number 

 of oil globules ; if the temperature is further increased the protoplasm 

 becomes green and then granular, and is detached from the wall of the 

 valves, which is a sure sign of the death of the frustule. 



B. Cold. When the diatoms are submitted to a temperature 

 of o C, to all appearances they scarcely appear to suffer at all, so long 

 as the medium remains liquid, but should it happen to freeze the diatoms 

 with very few exceptions succumb. 



2. Effect of Dessication. Experiments have shown that diatoms 

 keep alive as long as they remain humid, but they die directly they are 

 quite dessicated. 



3. Effect of Light. The rays which most favour the cultivation of 

 fresh water diatoms are the yellow, blue, and green, arranged in order 

 of merit. White rays are much less favourable. Semi-darkness is pre- 

 ferable to a flood of light, though the latter favours the prolific growth of 

 green algae. 



4. Effect of Heat. Cold considerably suspends or retards the progress 

 of cultivations ; at a temperature between 5 and io C. it takes from 

 10 to 12 days before the planted macerations show any well-pronounced 

 traces of cultivation; between io and 15 C. it only requires eight 

 days, and between 15 and 20 C. merely four to five days. 



5. Effect of Chemical Reagents. Chemical reagents may be nutri- 

 tive, indifferent, or toxic. By a toxic reagent Dr. Miquel means every 

 chemical body which, when added to the maceration in a less proportion 

 than one part to 1,000 parts by weight of the total quantity of cultivation 



