134 SOURCES OP HEAT AND COLD. 



sheaves or blocks of running tackle* on board of ships ; of ropes, pass- 

 ing rapidly over a gunwale, as when a whale is harpooned ; friction in 

 the boring of cannon and muskets j of a rope running rapidly through 

 the hand ; of the hand rubbed on a stair rail, or on one's woolen coat 

 sleeve ; all these and many others are instances of heat evolved on 

 this principle. 



The rotary match box gives sparks by the collision of a rapidly revol- 

 ving steel with flint, and a similar instrument called the steel mill, was 

 used to give light in coal mines before the invention of the safety lamp. 



An iron bar grows hot enough, by vigorous hammering, to kindle 

 shavings, and lead will by the same treatment kindle phosphorus. 



Wood, in rapid revolution, " may be carbonized throughout the 

 circle of contact, by holding against it another piece properly sharp- 

 ened, and one cork rubbed against another will become hot enough to 

 kindle phosphorus. "f A disk of soft iron rapidly revolving by ma- 

 chinery, will easily cut in twof the hardest steel saw plate, or the 

 best file. 



(g.) Vital action. This is evidently a source of heat, although 

 in a way not perhaps fully understood. There can be no doubt that 

 oxygen, acting in respiration, is an important agent in producing and 

 sustaining it ; it appears probable also that secretion, connected with 

 the influence of the nerves, is concerned, and some facts countenance 

 the opinion that galvanic agencies are not dormant. 



Whatever may be usefully said on the latter subject, belongs to a 

 more advanced stage of this work. 



II. THE SOURCES OF COLD. 



1. Evaporation, 



2. Rarefaction, 



3. Chemical action. 



1. Evaporation. The general facts on this subject have been al- 

 ready stated. Whenever a body passes to the aeriform state, it ab- 

 sorbs heat to turn it into vapor, and thus cools the contiguous bodies. 

 Sensible cold is produced by the evaporation of water, more by that 

 of alcohol, and most of all by that of ether or carburet of sulphur, 

 or liquid sulphurous acid, whether measured by our organs or by the 

 thermometer. We have already seen that water is frozen by the 

 evaporation of ether, both in the exhausted receiver of the air pump, 

 and in a tube in the atmosphere. The mercury in a thermometer 

 ball, wet with water and having a current of air blowing upon it, 

 will fall 5 ; if with alcohol, 12, and if with ether, 30. -Murray. 



* See Lt. Glynn in Am. Jour. Vol. XIV, p. 196, and Capt. Parry's 2d Voyage, New 

 York Ed. p. 212. " The weight of the ice every moment increasing, obliged us 

 to veer on the hawsers, whose friction was so great as nearly to cut thtough the bit 

 heads, and ultimately set them on fire, so that it became requisite for people to at- 

 tend with buckets of water." Parry. 



t Dr. Hare. t See Am. Jour. Vol. VI, p. 336. 



