FUEL. Dr. Black divides fuels into-. five classes ; the first comprehends the fluid inflammable bodies ; the second,. peat or turf ; the third, charcoal of wood; the fourth, pit-coal charred ; and the fifth, wood, or pit-coal, in a crude state, and ca pable of yielding a copious and bright flame.
The fluid inflammables are considered as distinct from the solid, on this account, that they are capable of burning upon a wick, and become in this way the most manageable sources of heat ; though, on account of their price, they are never em ployed for producing it in great quanti ties; and are only used when a gentle de gree, or a small.quantity of heat, is suffi cient. The species which belong to class are alcohol and different oils.
The first of these, alcohol, when pure and free of water, is as convenient and manageable a fuel for producing mode rate or gentle heats as can be desired. Its flame is perfectly clean, and free from any kind of soot ; it can easily be made to burn slower or faster, and to produce less or more heat, by changing the size or number of th e wicks upon which it burns; for as long as these are fed with spirit, in a proper manner, they continue to yield flame of precisely the same strength.
The cotton, or other materials, of which the wick is composed, is not scorched or consumed in the least, because the spirit with which it is constantly soaked is in capable of becoming hotter than 174° Fahrenheit, which is considerably below the heat of boiling water. It is only the vapour that arises from it which is hotter, and this too only in its outer parts, that are most remote from the wick, and where only the combustion is going on, in consequence of communication and contact with the air. At the same time, as the alcohol is totally volatile, it does not leave any fixed matter, which, by be ing accumulated on the wick, might ren der it foul and fill up its pores. The wick, therefore, continues to imbibe the spirit as freely, after some time, as it did at the first. These are the qualities of al cohol as a fuel. But these qualities be long only to a spirit that is very pure. lf, on the contrary, it be weak, and con tain water, the water, being less volatile, does not evaporate so fast from the wick as the more spirituous part ; and the wick becomes, after some time, so much soak ed with water, that it does not imbibe the spirit properly. The flame becomes much
weaker, or is altogether extinguished. When alcohol is used as a fuel, therefore, it ought to be made as strong, or free from water, as possible.
Oil, although fluid like spirit of wine, and capable of burning in a similar man ner, is not so convenient in many re spects. It is disposed to emit soot ; and this applying itself to the bottom of the vessel exposed to it, and increasing in thickness, forms, by degrees, a foft and spongy medium, through which heat is not so freely and quickly transmitted. This was observed by IiInschenbroeck, in his experiments upon the expansions of metalline rods heated by lamps. It is true we can prevent this entirely by using very small wicks, and increasing the number, if necessary, to produce the heat required. Or we may employ one of those lamps, in which a stream of air is allowed to rise through the middle of the flame, or to pass over its surface with such velocity as to produce a more com plete inflammation than ordinary. But we shall be as much embarrassed in ano ther way, for the oils commonly used, being capable of assuming a beat greatly above that of boiling water, scorch and burn the wick, and change its texture, so that it does not imbibe the oil so fast as before. Some have attempted a re medy, by making the wick of incom bustible materials, as asbestos, or wire ; but still, as the oil does not totally eva porate, but leaves a small quantity of gross fixed carbonaceous matter, this, constantly accumulating, clogs the wick to such a degree, that the oil cannot as cend, the flames become weaker, and, in some cases, are entirely extinguished. There is, however, a difference among the different oils in this respect ; some being more totally volatile than others. But the best are troublesome in this way, and the only remedy is, to change the wicks often, though we can hardly do this and be sure of keeping always as equal flame.