72 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



of its withdrawal. This change, however, is not immediate. Its first 

 step is a mere depression of the vitality of the part, involving a partial 

 stagnation of the capillary circulation, diminution of sensibility, and 

 want of muscular power. But the continued action of cold on the sur- 

 face, not compensated by a sufficient generation of heat within, causes 

 the circulation of the part to be completely suspended, its small vessels 

 contract so that they become almost emptied of blood, its sensibility and 

 power of movement are destroyed, in a word, its vital activity is com- 

 pletely suspended. In such a state, a timely but cautious application 

 of warmth may produce the gradual renewal of the circulation, and the 

 restoration of the other properties of the part, which are dependent upon 

 that function ; but any abrupt change would complete the mischief which 

 the cold has begun ; and would altogether destroy, by the violence of 

 the reaction, the vitality which was only suspended, causing the actual 

 death of the part. Hence, when the extremities are frost-bitten, nothing 

 can be more injurious than to bring them near a fire ; whilst no treat- 

 ment has been found so safe and effectual as the rubbing them with snow. 



98. The influence of Heat upon Vital activity, is attested on a larger 

 scale, by the striking contrast between the dreary barrenness of Polar 

 regions, and the luxuriant richness of Tropical countries, where almost 

 every spot to which moisture is supplied teems with Animal and Vege- 

 table life. And the alternation of Winter and Summer in temperate 

 climates, may be almost said to bring under our own view the opposite 

 conditions of those two extreme cases. The effect of the withdrawal of 

 Heat is most obvious in the Vegetable kingdom ; since all its operations 

 are dependent upon a certain supply of that agent ; and in no case are 

 Plants possessed of the power of generating that supply within them- 

 selves, excepting in certain organs which do not impart it to the rest 

 of the structure. When the temperature of the air falls to the freezing- 

 point, therefore, we find all the operations of the Vegetable economy 

 undergoing a complete suspension ; yet a very trifling rise will produce 

 a renewal of them. It is not only in Evergreens, that the vital pro- 

 cesses continue to be performed to a certain extent during the winter ; 

 for there is abundant evidence that, even in the trunk arid branches of 

 trees unclothed with leaves, a circulation of sap takes place, whenever 

 there is even a slight return of warmth. In this manner, the leaf-buds 

 are gradually prepared during the milder days of winter, so as to be 

 ready to start forth into full development, with the returning steady 

 warmth of spring. 



99. The influence of Heat upon Vegetation is easily made apparent 

 by experiment ; in fact experimental illustrations of it, on a large scale, 

 are daily in progress. For the Gardener, by artificial warmth, is not 

 only enabled to rear with success the plants of tropical climates, whose 

 constitution would not bear the chilling influence of our winter ; but 

 he can also, in some degree, invert the order of the seasons, and produce 

 both blossom and fruit from the plants of our own country, when all 

 around seems dead. This process of forcing, however, is unfavourable 

 to the health and prolonged existence of the plants subjected to it; 

 since the period of repose, which is natural to them, is interrupted ; and 

 they are caused, as it were, to live too fast. The same result occurs, 



