740 PHYSIOLOGY. 



analysis shows, that the amount of oxygen present in the air 

 under ordinary circumstances, is not hable to appreciable varia- 

 tion, and we find, accordingly, — that Avhilst man and other ani- 

 mals vitiate the atmosphere by robbing it of oxygen gas in their 

 respiration, and by the various processes which are necessarily 

 carried on by them, plants purify it by restoring that vitally 

 necessary gas in their respiration. Thus we see that, '' the tAvo 

 great organised kingdoms of nature are made to co-operate in 

 the execution of the same design ; each ministering to the 

 other, and preserving that due balance in the constitution of 

 the atmosphere which adapts it to the welfare and activity of 

 every order of beings, and which would soon be destroyed were 

 the operations of either of them to be suspended. It is impossible 

 to contemplate so special an adjustment of opposite effects without 

 admiring this beautiful dispensation of Providence, extending 

 over so vast a scale of being, and demonstrating the unity of 

 plan upon which the whole system of organised creation has 

 been devised." 



In a like manner, plants purify the water in which they grow, 

 and render it habitable by animals. We all knoAv by early 

 experience, that if any kind of fish be placed in Avater in Avhich 

 no plants are grown, they Avill soon perish. This is the case, 

 because there is then nothing present in the Avater to destroy 

 the noxious matters Avhich are given off by them in their respi- 

 ration and other processes, and thus they perish by their own 

 action upon the medium in which they are placed. In Nature, 

 Ave always find plants existing with animal life in the Avater, so 

 that the injurious influence communicated by the latter to that 

 medium, is counteracted by the respiration of the former. This 

 compensating influence of plants and animals is beautifully il- 

 lustrated in our vivaria. Hence avc see the importance again, 

 in our sanitary arrangements, of bearing in mind these eflfects 

 of plants ; for Ave are taught by them, that it is absolutely neces- 

 sary, if Ave desire to maintain a large toAvn in a healthy state, to 

 set apart large areas and plant them freely, for by such means, avc 

 not only maintain the purity of the air, but also, at the same 

 time, afford spaces for exercise and recreation. 



Let me here briefly notice a Avidely-spread notion, that plants, 

 when groAvn in rooms Avhere there is but little ventilation, and, 

 therefore, especially in our sleeping apartments, have an injurious 

 influence upon the air contained in them. This idea has arisen 

 from the circumstance of plants, Avhcn not exposed to solar light, 

 having a contrary effect upon the atmosphere to that Avhich 

 they have Avhcn submitted to its influence ; that is to say that 

 they then absorb oxygen and give off carbonic acid gas, instead 

 of absorbing car})onic acid gas and giving oft" oxygen. The 

 amoimt of carbonic acid gas, hoAvever, Avhich is then given off 

 by plants is so extremely small, that it can have no sensible 



