122 HEAT OR CALORIC. 



ed by a brass syringe, furnished at one end with a little chamber, 

 containing tinder, agaric, or other combustible, which is heated by 

 the compression produced by the quick stroke of the piston, so that 

 the combustible, on being suddenly brought to the air, by the turn- 

 ing of a key, took fire. More recently, the combustible is contained 

 in the piston itself, which, after the stroke, is quickly withdrawn from 

 the tube. The instrument is now made of glass, which enables one 

 to see the flash. 



(k.) Changes of capacity for caloric have an intimate connexion 

 with the regulation of natural and artificial temperature. The me- 

 dium heat* of the globe is usually placed at about 50 of Fahr. and 

 is found, as has been heretofore believed, at about 1000 feet below the 

 surface of the ground. 



Medium heat of the atmosphere at New Haven, about 50. f 

 " " " the Torrid zone, 70 to 80. 



" " " moderate climates, 50 to 52. 



" " " near the polar regions, about 36. 



The extremes of the globe are from about 50 sometimes 70 

 to 100, 105, 110; and even 120, or perhaps in some situa- 

 tions, still more. 



The extremes of artificial temperature* are much greater, from 

 91, to 35127, (Henry.) which is the highest estimated heat, but 

 we know that it is not the highest heat that has actually been produc- 

 ed. We have no measure for it, and probably can never have any 

 other than the effects which such heats produce in fusion, &c. The 

 real zero has never been discovered. { 



( /.) Freezing mixtures act by enlargement of capacity. A solid, 

 as already observed, is always one ingredient in these compositions ; 

 it becomes fluid by uniting, chemically, with some other agent, and 

 thus absorbs heat and produces cold. Salts and acids, as Glauber's, 

 eight ounces, and muriatic acid, five ounces, are most commonly em- 

 ployed, and sink the thermometer from 50 to 0. When both in- 

 gredients are solid, the mixture is still more powerful, as in the case 

 of muriate of lime and snow ; and of muriate of soda and snow ; by 

 the former, mercury is frozen. Snow, or pounded ice, two parts, and 

 common salt, one part, depress the thermometer from 50 to 5. 



The mere solution of a salt in water produces cold. Nitre, in 

 large quantities, added to water, sinks the thermometer 17 ; ni- 



- Should the views of Prof. Cordier, as to the increasing heat of the interior of 

 the earth, be established, the result stated in the text cannot be correct ; but it will 

 require numerous and often repeated observations, extending to many countries, 

 and through many years, to establish a conclusion so extraordinary See Am. Jour. 

 Vol. 15. p. 109. 



t Pres. Day, in Trans, of Conn. Acad. 



t We think it useless to reiterate the fruitless discussions on this subject ; they 

 may be found in all the larger chemical works. It is evident that no reliance can 

 be placed upon the results, widely discordant as they are. 



For a more copious table of freezing mixtures, see p. 136. 



