220 



NEW ENGLAND FARMER. 



For the New EngUitxd Farmer. 



THE RADIATION AND CONDUCTION OF 

 HEAT. 



Mr. Cole : No meteorological phenomena arc so 

 pregnant with interest, or of such preeminent import 

 to the farmer, as the transit of heat from one sub- 

 stance to another, and from terrestrial bodies to the 

 atmosphere. And yet, speaking generally, there is 

 scarcely a cultivator who can clearly cxjilain their 

 nature, or definitively state how they are accom- 

 plished. Many most ridiculous notions concerning 

 them are rife in the world of horticulture, a few of 

 which have even been supposed to receive confirma- 

 tion from professedly scientific sources ; so that it is 

 incumbent on some one to place the subject in so 

 simple a light, that no further mistakes may be jus- 

 tifiable. 



Philosophers have long since determined that 

 changes of temperature, in common Avith all other 

 procedures of nature, are regulated by certain un- 

 varying laws. Of these, one of the most prominent 

 is the transition of heat from a warm body to any 

 cold one that may be contiguous, till the temperature 

 is equalized. Now, although air is too seldom re- 

 garded as a refined and svibtile substance, such is its 

 imquestionablc nature. Hence the process termed 

 radiation, is, in point of fact, though to a limited 

 e.Ktent, a kind of conduction ; the small particles of 

 matter of which the atmosphere is composed being 

 the media through which, when brought into contact 

 with objects on the earth's surface, their heat is ab- 

 sti-actcd. 



The above position does not, however, wholly hold 

 good; since heat is said to be capat)le of pervading a 

 vacuum, and matter cannot consec^uently be a neces- 

 sary auxiliary to its dispersion. For all practical 

 purposes, therefore, and likewise to facilitate the in- 

 culcation of the precepts of science, a very proper 

 distinction is made between radiation and conduc- 

 tion. Both are the result of expansion — one of the 

 most striking properties of heat ; but the former is 

 the means through which the temperature of a body 

 is lowered by diffusing itself into a colder «*>, while 

 the latter term is applied to the passage of heat from 

 a Warm solid substance to a cold one, when placed in 

 immediate contact therewith. 



Radiation, as the very word implies, is the diver- 

 gence of a number of heated atoms, in the forms of 

 rays, from a body thoroughly warmed ; or, as some 

 assert, it is the mere emission of calorific rays, causing 

 a greater or less undulation in the constituents of 

 the atmosphere, whereby an increase of temperature 

 is occasioned. Thus the sun radiates heat perpet- 

 ually, some of its rays being transmitted through our 

 air to the earth, by Avhich they are received, and 

 from which, during the absence of that brilliant 

 hmiinary, they again emanate in a similar manner. 

 It is very frequently confounded with refraction, 

 which is quite another process, and most markedly 

 different ; radiation being the simple issue of heat 

 from any surface, and refraction the interception of 

 i) - rays by an interposed screen, and their re-radia- 

 tion from thence towards the point from whence they 

 ori'^inally proceeded. For example, the earth refracts 

 many of the rays that reach it from the sun ; and it 

 ij chiefiy from this cause that the geniahty of our 

 atmosphere arises. But it also imbibes other rays, 

 and the radiaiioti of these serves to prevent a coldness 

 during the night, that would be wholly destructive 

 to both animal and vegetable life. 



Having thus faid the basis of the application of 

 artificial coverings to plants on the principles of 

 science, the following hints on protection will not be 

 misapprehended. Plants, being formed of earthy and 

 atmo.siiheric elements, combined and concentrated 



according to their peculiar powers, have, in propor- 

 tion to their porosity, and with a slight reduction on 

 account of their vital energy, the same tendency to 

 radiation as soil. Unless, then, this bo duly inter- 

 ru})ted, the more susceptible kinds must be subjected 

 to irrenicdiable injury ; whereas, if radiation can bo 

 effectually suspended, they will be perfectly safe in 

 the most severe weather. 



It follows from these premises, that the old method 

 of supplying fire heat to plant-houses, Avhcrc the sole 

 design is to exclude frost, is radically wrong ; indeed, 

 it is a positive and total waste, expended, too, on an 

 object that cannot sometimes be realized by such a 

 practice, and the failure of which is more or less 

 likely to be fatal to the plants. If it be demanded 

 what I Avould substitute for it, I vrould suggest — 

 prevent the esca2)e or radiation of heat, and there will 

 be no need to essay the supply of a deficiency that 

 does not exist. 



If these hints on radiation have received half the 

 attention -which their subject demands, the reader 

 will be ready patiently to pursue the investigation as 

 it concerns conduction. I have distinguished the 

 two processes by showing that the first phrase char- 

 acterizes the transmissions of heat to the superin- 

 cumbent air, while the topic now to be discussed 

 embraces the deprivation of temperature which a 

 body suffers on being touched by a colder one. On 

 the extent to which the latter circumstance is ob- 

 viated, the merits of any system of covering plants, 

 whether in houses, frames, or the open ground, must 

 to a very great degree be wholly dependent. 



It is notorious to every inquiring person, that heat 

 is much more speedily dissipated when the substance 

 containing it is in direct connection with some less 

 highly heated material. But the more humble mem- 

 bers of our profession find it difficult to comprehend 

 or believe this fact; let us then, adduce an easy 

 illustration. If a bar of cold iron (which, perhaps, 

 is one of the most rapid conductors of heat) bo 

 placed within an inch of a person's hand, the dim- 

 inution of caloric, (heat,) Avhich its vicinity occa- 

 sions, is scarcely perceptible ; but if the same sub- 

 stance be grasped, or laid on the hand, the abstraction 

 of heat is so sensibly felt, that without a strong 

 efi'ort of volition, the iron would be immediately 

 dropped. Again, the atmosphere of an apartment 

 may be exceedingly comfortable as regards temper- 

 ature, and altogether consonant with the appetencies 

 of a sensitive person, while by pressing the hand 

 against glass of which the outer side is exposed to a 

 severe external air, considerable pain will be expe- 

 rienced from the loss of caloric consequent on such 

 an act. 



I have chosen these commonplace examples the 

 more completely to demonstrate the position, although 

 the experience of the cultivator of extensive practice 

 might at once decide this point. 'There is the most 

 apposite analogy, in respect to heat, between an indi- 

 vidual in a confined room, and a plant in a sort of 

 tent-like, or any other protective frame. Both may 

 be kept from perishing by frost if situated in the 

 middle of the area, biit both must undergo the loss 

 of those parts which are in contact with a thin cov- 

 ering closely communicating with the outer air, if 

 frost be sufficiently severe. Proofs of this have often 

 been furnished in the case of plants whose shoots 

 touched the glass or mats by which they were sur- 

 rounded. 



Herein, therefore, lies the art of protecting plants. 

 They must first be envtloped in a material which is 

 known to be an impcrloct radiator of heat, so that 

 their own temperature may, for the most part, be 

 retained within or around them. And secondly, that 

 material should be so disposed that no part of it be 

 nearer than about two or three inches to the exterior 

 shoots. The importance of confining and tying i*, 



