92 



FARMERS' REGISTER, 



No. 2 



and consequently its volume, diminished. The 

 act of striking, or rubbing hard bodies together, 

 produces the same effect; tiie portion of caloric, 

 which is in either case set free, acts as heat. 



The temperature of bodies can he lowered, or 

 elevated, by placing them in contact with other 

 bodies more or less hot than themselves. Ihe 

 fluid of heat will pass (rom one to the other, and 

 produce an equilibrium in their state, according to 

 their respective capacities for ca!oric,vvhich enable 

 them to absorb unequal quantities of it. 



All bodies in their natural state contain a deter- 

 minate portion of caloric; but when their densi-. 

 ty undergoes a change, by the variations of tem- 

 perature^to which they are exposed, they lose or 

 absorb caloric, in proportion to their contraction or 

 dilation. The gases, which become solid by en- 

 tering into coiTobinations, the vapors which are 

 condensed, the solids which are contracted, impart 

 to the air a portion of their caloric, which be- 

 comes heat; whilst all these bodies, on receiving 

 heat from the air, are dilateJ. 



The phenomena of composition, and decompo- 

 eition, which uninterruptedly renew the siirtace of 

 our o-lobe, give rise every moment to the emission 

 or absorption of caloric. Two substances, enter- 

 ing into combination, form a compound which 

 peThaps requires more or less caloric than is con- 

 tained in the two component principles; and then 

 either heat or cold is necessarily produced during 

 the operation. Those gases, wdiich become solid 

 by entering into combination, part with their calo- 

 ric whilst undergoing the change, and thus produce 

 heat. In combustion, where oxygen is the prin- 

 cipal agent, there is a constant disengagement of 

 caloric,%ecause that gas, in general, forms solid or 

 liquid compounds with combustible substances; 

 and it gives out a portion of the caloric which pre- 

 served it in its gaseous state. 



These principles established, we can easily ex- 

 plain a part of the effects produced upon vegeta- 

 tion by the variations of temperature. 



The changes of temperature, experienced by the 

 atmosphere in the course of a year, are so great, 

 as to cause some liquids to pass alternately either 

 to the solid or aeriform state, and some solid bo- 

 dies to become liquid. The natural effect of heat 

 upon these bodies is, by dilating them, to weaken 

 the force of cohesion which unites their molecules, 

 and, by facilitating the action of chymical affinity, 

 to enable them to enter into combination with fo- 

 reign bodies. Thus heat renders the juices of 

 plants more fluid, and facilitates their circulation 

 through the cells and capillary vessels; and, by 

 giving activity to the suckers of roots, enables 

 them to draw from the earth the juices necessary 

 ibr their nourishment. 



Above a certain temperature, heat, by promo- 

 ting evaporation, causes the juices of plants to 

 become thickened and dried in their organs, and 

 thus vegetation is arrested, and lifii sus|)ended. 

 This effect always takes place during great heats, 

 when neither rain, dew, nor irrigation can sutii- 

 ciently repair the loss occasioned by evaporation. 

 This effect would be more frequent, if provident 

 nature did not employ means to moderate the ac- 

 tion of hetit. 



The first of these means is the transpiration of 

 the vegetables themselves, which cannot take 

 place without carrying off a large portion of heat, 

 and thus preserving the transpiring body at a 



temperature below that of the air. The second 

 means is found in the organization of leaves, 

 which are the only parts of a plant where trans- 

 piration takes place. That sudiice of leaves which 

 is exposed to the direct rays of the sun is covered 

 by a thick epidermis, which resists the calorific 

 rays. In herbaceous plants, as in the stalks of 

 grasses, this covering is composed principally of 

 silex. In otlier plants it is analogous to resin, 

 wax, gum, or honey; vvhilst the epidermis, which 

 covers the oi)p()si!e sides of the leaves, is fine and 

 transparent. It is by tiiis, that transpiration and 

 the absorption of nourishment from the atmos- 

 phere are carried on. If we should reverse the 

 order of things, and present the under surface of a 

 leaf to the rays of the sun, we should very soon 

 see that it would make great efforts to resume its 

 natural position. 



When a plant is dead, or rather when an annual 

 plant has fulfilled its desti^ly, giving assurance of 

 its reproduction by the formation of its fruit, the 

 action of heat and of the other chymical agents is 

 no longer modified by any of the causes of which 

 I have just spoken, and the plant receives their 

 impression in an absolute and unmodified manner. 

 When the temperature of the atmosphere sinks 

 below a certain point, the fluids in plants become 

 condensed, the movement of the juices is retarded, 

 the activity of their organs languishes, and is at 

 length suspended, until restored by the return of 

 hear. The action of the atmosphere upon plants, 

 when deprived of its due proportion of heat, is 

 however modified by the emission or disengage- 

 ment of caloric, which is always given out when 

 liquids are condensed, or solids contracted; and 

 this occasions the temperature of plants, during 

 the winter, to be always a little higher than that 

 of the atmosphere. 



It sometimes happens that the temperature of 

 the atmosphere sinks so low, as to produce fatal 

 effects upon plants, by fi-eezing their sap, and thus 

 occasioning their death. This effect does not al- 

 ways depend upon the intensity or degree of cold 

 to which they are exposed, but upon particu- 

 lar circumstances. I have seen olive trees resist a 

 temperature of 22'^. 2 Fahrenheit, and perish from 

 that of 28°. 6; because in the last case the snow, 

 which had collected upon the branches of the 

 trees during a night, was dissolved the following 

 day by the heat of sun, and the wet tree was ex- 

 posed during the succeeding night to the action of 

 28". 6. There is nothing more dangerous for corn 

 and grasse.s, than those fi'osts which follow imme- 

 diately afler a thaw, because the still wet plants, 

 not being deeply rooted in the ground pulverized 

 by the frost, have no means of deftjnding them- 

 selves from the effects of the cold. 



3. Sennebier was the first to admit that the in- 

 fluence of light was hurtful to the germination of 

 seeds. Ingenhouz confirmed this opinion by ac- 

 tual experiment; but M. de Saussure, who caused 

 grains to germinate under two receivers, the one 

 opaque, and the other transparent, is convinced 

 that germination took place in both cases at the 

 same time; but that the subsequent vegetation 

 was more rapid and vigorous under the transpa- 

 rent, than under the opaque receiver. It is easy 

 to reconcile these opinions and results, though ap- 

 parently so contradictory, by separating the ac- 

 tion of heat fi-om that of light. As plants trans- 

 pire very little during their first stage of germina- 



