LIFE IN OTHER WORLDS JONES 147 



surest -way of killing any form of life is to subject it to a high tempera- 

 ture. We boil water of doubtful purity because germs, which may be 

 resistant to many destructive agents, cannot long survive at the tem- 

 perature of boiling water. The more complex the form of life the more 

 susceptible it becomes to any destructive agency because, if the proto- 

 plasm of essential cells is destroyed, the proper functioning of the 

 organism is upset and the death of other cells, and eventually of the 

 whole organism, as a rule results. 



Low temperatures are also effective in destroying life, though some 

 forms of life can withstand extreme cold for long periods. In such 

 cases, however, there seems to be a state of suspended animation, in 

 which all vital processes cease until the temperature is raised. Life 

 becomes arrested or latent; the chemical structure is uninjured but 

 chemical change is stopped. We cannot conceive that, if such condi- 

 tions prevailed on any world, life could develop. There cannot possibly 

 be development if all vital processes are suspended. We can therefore 

 conclude that a world where there is a very high or a very low tem- 

 perature is unlikely to be the home of any sort of life. 



Satisfactory conditions of temperature are not in themselves suffi- 

 cient. A further essential condition would seem to be the presence of 

 water, either in the liquid form or as vapor. It is by imbibing water 

 containing the chemical substances on which they feed that cells grow 

 and divide. Neither seeds nor spores will germinate in soil that is 

 devoid of moisture, and water is an essential constituent of the tissues 

 of both animal and vegetable life. In the absence of moisture life, 

 when not actually destroyed, becomes latent. Most forms of life with 

 which we are familiar are dependent for their activity upon the pres- 

 ence of free oxygen, though the absence of oxygen does not seem to be 

 necessarily fatal to every form of life. Carbon dioxide is favorable for 

 the existence of vegetable life. Many other gases are poisonous to life : 

 ammonia, clilorine, carbon monoxide, and sulphuretted hydrogen all 

 have a marked toxic action. Though their presence in the atmosphere 

 of any world would not necessarily prove conclusively that there could 

 be no life on it, it would provide some evidence against its probability. 



The presence in the atmosphere of any world of water vapor, oxygen, 

 and carbon dioxide would therefore be evidence in favor of the possi- 

 bility of the existence of life ; the absence of these, combined with the 

 presence of poisonous gases, would be evidence against the possibility 

 of its existence. The absence of any atmosphere would necessarily be 

 conclusive proof that life could not exist. 



We can at once rule out of consideration the Sun and all other stars. 

 The temperatures of the coolest stars are so high that only the simplest 

 compounds can exist in their outer layers. The complicated molecular 



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