CHARACTERS OF LIVING MATTER. n 



live, and is resolved into more higlily-oxidated compounds of 

 its elements. 



Thus an individual living body is not only constantly 

 changing its substance, but its size and form are undergoing 

 continual modifications, the end of which is the death and 

 decay of that individual ; the continuation of the kind being 

 secured by the detachment of portions which tend to run 

 through the same cycle of forms as the parent. No forms of 

 matter which are either not living, or have not been derived 

 from living matter, exhibit these three properties, nor any 

 approach to the remarkable phenomena defined under the sec- 

 ond and third heads. But, in addition to these distinctive 

 characters, living matter has some other peculiarities, the 

 chief of which are the dependence of all its activities upon 

 moisture and upon heat, within a limited range of tempera- 

 ture, together with the fact that it usually possesses a certain 

 structure, or organization. 



As has been said, a large proportion of water enters into 

 the composition of all living matter ; a certain amount of dry- 

 ing arrests vital activity, and the complete abstraction of this 

 water is absolutely incompatible with either actual or poten- 

 tial life. But many of the simpler forms of life may undergo 

 desiccation to such an extent as to arrest their vital manifes- 

 tations and convert them into the semblance of not-living 

 matter, and yet remain potentially alive ; that is to say, on 

 being duly moistened they return to life again. And this 

 revivification may take place after months, or even years, of 

 arrested life. 



The properties of living matter are intimately related to 

 temperature. Not only does exposure to heat sufficient to 

 decompose protein matter destroy life, by demolishing the 

 molecular structure upon which life depends ; but all vital 

 activity, all phenomena of nutritive growth, movement, and 

 reproduction, are possible only between certain limits of tem- 

 perature. As the temperature approaches these limits the 

 manifestations of life vanish, though they may be recovered 

 by return to the normal conditions ; but, if it pass far beyond 

 these limits, death takes place. 



This much is clear ; but it is not easy to say exactly what 

 the limits of temperature are, as they appear to vary in part 

 with the kind of living matter, and in part with the con- 

 ditions of moisture which obtain along with the temperature. 

 The conditions of life are so complex in the higher organisms, 

 that the experimental investigation of this question can be 



