CONDUCTION OF HEAT. 183 



worse conductors than those composed of cotton or linen. A flannel shirt 

 more effectually intercepts heat than a cotton or a linen one ; and whether 

 in warm or in cold climates, attains the end of clothing more effectually. 



If we would preserve ice from melting, the most effectual means would be 

 to wrap it in blankets, which would retard, for a long time, the approach of 

 | heat to it from any external source. 



Glass and porcelain are slow conductors of heat, and hence maybe explain- 

 ed the fact, that vessels formed of this material are, frequently, broken by sud- 

 denly introducing boiling water into them. If a small quantity of boiling water 

 be poured into a thick glass tumbler, the bottom, with which the water first 

 comes into contact, is suddenly heated, and it expands ; but the heat, passing 

 very slowly through it, fails to affect the upper part of the vessel, which, there- 

 fore, undergoes a corresponding expansion : the lower part enlarging, while 

 the upper part remains unaltered, a crack is produced, which detaches the bot- 

 tom of the tumbler from the upper part of it. 



In the construction of an ice-house, the walls, roof, and floor, should be sur- 

 rounded with some substance which conducts heat imperfectly. A lining of 

 straw-matting, or of woollen-blankets, will answer this purpose. Air being a 

 bad conductor of heat, the building is, sometimes, constructed with double 

 walls, having a space between them. The ice is thus surrounded by a wall 

 of air, as it were, which is, in a great degree, impenetrable by heat, provided 

 no source of radiation be present. Furnaces, intended to heat apartments, 

 should be surrounded with non-conducting substances, to prevent the waste of 

 heat. 



When wine-coolers are formed of a double casing, the space between rnay 

 be filled with some non-conducting substance, such as powdered charcoal, or 

 wool, or it may be left merely filled with air. 



The practical application of non-conduction is illustrated in the construction 

 and management of the boilers and steam-pipes of steam-machinery. 



In places where fuel is expensive and consumed in great quantity, every 

 possible expedient that can conduce to its economy is resorted to. In Corn- 

 wall, where very powerful engines are worked for the drainage of the mines 

 and the preparation of the ore, and to which fuel has to be carried from a con- 

 siderable distance, the boilers are surrounded by a hollow casing, stuffed with 

 saw-dust. This is found to be a nearly perfect non-conductor of heat. All the 

 pipes which conduct steam to the cylinders are similarly coated. The conse- 

 quence of this is, that the boiler-houses, notwithstanding the large furnaces 

 continually burning in them, are extremely cool rooms, and in summer are 

 much cooler than the external atmosphere. The steam cylinders are also, 

 sometimes cased in wood. 



In the machinery used in the British steamships, it has been the practice 

 to invest the boiler with a coating of patent felt and to cover the great steam- 

 pipe in the same manner. This non-conducting coating prevents the con- 

 stant waste of steam by the condensation produced by radiation. 

 I Charcoal in powder is a good non-conductor of heat, and is sometimes used 

 to protect ice from fusion. 



Fresh provisions are sometimes exported to distant places enveloped in ice. 

 In this case it would be advantageous to envelope the ice itself in a casing of 

 saw-dust. 



In concluding these discourses on heat, it may be proper to enumerate 

 the most ordinary sources of this principle. They may be stated as fol- 

 lows : 



1 . Solar Light. The sources from which heat might, by possibility, be ra- 

 diated toward the earth from distant regions of the universe are, 1st, the sun ; 



