8 THE AQUAVIVARIUM. 



Anacharis alsinastrum more vigorous than ever. 

 The habits of our plants and animals in this respect 

 must be studied. As a rule, it is safer never to let 

 the temperature fall below 40 or to rise higher 

 than 65 or 70. Hence the importance of selecting 

 proper situations for permanent tanks. If they 

 are exposed to the north, they must be looked 

 after in the winter, lest they get too cold ; and if 

 to the south, in summer, lest they get too hot. 



The world is supplied with light from the same 

 source as heat ; at the same time, it is most im- 

 portant to distinguish between the action of these 

 two agents : we can always command heat but not 

 light. If we grow plants in a dark cellar, we 

 shall find, although they have plenty of heat, that, 

 for the want of light, they become pale and die. 

 Plants exposed to a northern aspect will die, whilst 

 those in the south will flourish and produce their 

 flowers and fruit : this arises from the action of 

 light. Not only do our large water-plants grow best 

 in the light, but a thousand minute forms of plants 

 start into existence when the Aqua vivarium is ex- 

 posed to the light of the sun. This wonderful agency 

 of light seems to arise from its chemical action on 

 the material of which the plant is composed : this 

 action is the same as that which takes place when 

 a sun-picture is produced. Just in proportion to 

 the light of the sun is the change produced on the 

 photographic paper ; and just in proportion to the 

 light of the sun is the growth of vegetable matter 

 in the Aquavivarium. We have thus a powerful 

 means in our hands of increasing or decreasing the 

 growth of the plants which we are cultivating. 



