52 Hood on warming Buildings hy Hot Water. 



ployed in practice ; and it exhibits not only a considerable know- 

 ledge of the principles of science, but also great ingenuity in their 

 application." (p. 133.) The high-pressure hot-water apparatus 

 of Mr. Perkins is described at length, and the great danger at- 

 tending its use pointed out ; with, however, a redeeming para- 

 graph at the end. We quote the following, as well for the sake 

 of gardeners as of general readers : — 



" 183. In consequence of the intense heat of these pipes, it is sometimes 

 found that rooms which are heated by them have the same disagreeable and 

 unwholesome smell which results from the use of hot-air stoves and flues. In 

 realit}', the cause is the same in both cases ; for it arises partly from the de- 

 composition of the particles of animal and vegetable matter that continually 

 float in the air, and partly from a change which atmospheric air undergoes by 

 passing over intensely heated metallic surfaces.* Fi'om some experiments 

 recorded in the Philosophical Transactions of the Royal Society, made with a 

 view of ascertaining the eiFect produced on the animal economy by breathing 

 air which has passed through heated media, it appears that the air which has 

 been heated by metallic surfaces of a high temperature must needs be exceed- 

 ingly unwholesome. A curious circumstance is related in reference to these 

 experiments, which is illustrative of this fact : — 



" * A quantity of air, which had been made to pass through red-hot iron 

 and brass tubes, was collected in a glass receiver, and allowed to cool. A large 

 cat was then plunged into this factitious air, and immediately she fell into con- 

 vulsions, which, in a minute, appeared to leave her without any signs of life. 

 She was, however, quickly taken out, and placed in the fresh air, when, after 

 some time, she began to move her eyes, and, after giving two or three hideous 

 squalls, appeared slowly to recover. But on any person approaching her, she 

 made the most violent efforts her exhausted strength would allow to fly at 

 them, insomuch that in a short time no one could approach her. In about 

 half an hour she recovered, and then became as tame as before.' 



" 184. The high temperature of these pipes, and the intensity at which the 

 heat is radiated from them, has sometimes been urged as an objection against 

 this invention, when applied to horticultural purposes; because any plants which 

 are placed within a certain distance of them are destroyed. Although, no 

 doubt, this effect really takes place, it can be easily avoided with proper care ; 

 for, as radiated heat decreases in intensity as the square of the distance, it only 

 requires that the plants should be placed farther off" from these pipes than 

 from those which are of a lower temperature. In comparing the effect of two 

 different pipes, if one be four times the heat of the other (deducting the 

 temperature of the air in both cases), the plants must be placed twice as far off" 

 from the one as from the other, in order to receive the same intensity of 

 heat from each. The only inconvenience, therefore, is the loss of room, 

 which, in some cases, may not be of much importance. But a more serious 

 objection, by far, appears to lie in the inequality of temperature which any 

 building heated by these pipes must have, in consequence of their being so 

 very much hotter in one part than in another. This difference of temper- 

 ature between various parts of the same apparatus has already been stated to 

 amount, in some cases, to as much as 200° or 300° ; varying, of course, with 

 the length of pipe through which the water passes. From what has been 

 stated in Chapter IV., it will also be observed that, owing to the smallness of 

 these pipes, this kind of apparatus cools so rapidly when the fire slackens in 



" * The exact nature of this change which the air undergoes has not been 

 ascertained i but, whatever be the chemical alteration which occurs, a 

 physical change undoubtedly takes place, by which its electrical condition is 

 altered. 



