208 



WRITINGS OF JOSEPH HENRY. [1855- 



that other changes than those of a mechanical character are 

 produced; but on this subject much research is required, and 

 every intelligent farmer may add important materials to our 

 stock of knowledge by carefully recording the observations 

 he may make relative to the reduction of temperature, and 

 its continuance, by which certain plants are destroyed. 



It is shown by repeated observations that alternations of 

 freezing and thawing are more hurtful to the tender plant 

 than a uniform continuation of cold ; whether this is pro- 

 duced by an action analogous to that we have described in 

 reference to the water-pipe, or is due in part to other changes 

 we are unable to say. When however the sap of a plant 

 killed by frost is examined with a microscope, we find in it 

 portions of destroyed tissue. It has also been observed 

 that air may sink a few degrees below the freezing point 

 without injury to the plant, provided the air at the time 

 be very dry. It would seem from this that the freezing of 

 the vapor and the production of the minute crystals which 

 constitute hoar frost are in a degree essential to the effect. 



As a general deduction from chemical and mechanical 

 principles, we think no change of temperature is ever pro- 

 duced in plants without the concurrence of actions such 

 as here indicated. Hence, in mid- winter, when all vege- 

 table functions are dormant we do not believe that any heat 

 is developed by a tree, or that its interior differs in tem- 

 perature from its exterior further than it is protected from 

 the external air. The experiments which have been made 

 on this point, we think, have been directed by a false anal- 

 ogy. During the active circulation of the sap and the pro- 

 duction of new tissue, variations of temperature belonging 

 exclusively to the plant may be observed ; but it is incon- 

 sistent with general principles that heat should be generated 

 where no change is taking place. 



Effect of cold on animals. — All animals, so long as life con- 

 tinues, generate heat, and have temperatures peculiar to 

 themselves. In the higher class of air-breathing animals 

 this temperature varies within comparatively slight limits 

 under the influence of motion, rest, or of external circum- 



