Science Lecture.

Wairarapa Standard, Volume XIX, Issue 1847, 21 June 1886, Page 3

Science Lecture.

Lecture Vl—He.h. By Alex. Purdie, M.A. One ol the commonest iff ids of heat is to cause change of state in the mutter subjected to its action. Heat is in fact one of the chemist’s most powerful assistants. Many substances solid at ordinary temperatures become, under the influence of heat, first liquid, and them gaseous, if it does not happen in the case of compounds that their elements are driven apart by heat. We are accustomed to nse the terms melting or fusion to denote the passage from the solid to the liquid state, and very many substances may be thus i changed by heat. Exp* 19. The familiar substance wax, will melt with very little heat, soon solidifying again when the heat is removed. Exp. 20. The metal lead can be fused or melted at comparatively a low temperature. These two instances will suffice to show the transition from the solid to the liquid state under the action of beat. Taking next the passage from the liquid to the gaseous state ; every day in our houses we may see water changed from the liquid state to gaseous steam, by the process of boiling, that is by the application of heat. Exp. 21. Similarly, alcohol or kerosene may be driven off as gases by beating them in a wide mouthed dish, taking cate not to let the flame touch the heated liquid. In the case of alcohol or of kerosene, the gas pro duced is inflammable, and the presence of the gas may be shewn by bolding a light some distance above the dish, when tbe inflammable gas rising will be kindled at the flame. Be careful to remove combustibles from tbe neighbourhood of tbe dish yon are beating. Exp. 22. Some very volatile solids can be driven off in the gaseous stats with but slight application of heat; for instance, take tbe experiment already performed in the first lecture, the sublimation of iodine. This ■nbetance slowly volatilizes at ordinary temperatures, and passes off as a gas readily, with little heat, producing beautiful violent fumes. In exp. 21 I referred to the evaporation or boiling of water. It is usual to employ the first of these terms to denote the gradual passage ol water into tbe gaseous state when freely exposed to tbe air at ordinary tores, while the second term is used lor the process by which water is made to pais very rapidly into the gaseous state by heating strongly. It is easy to bring back to the liquid state, or condense tbe .water thus driven off as a gas. Exp 23. Tbe process is called distillation, and is sometimes used to purify water, ae many imparities not being rendered gaseous by heat are left behind, while the watei passes off pare. To distil water cause tin steam proceeding from the delivery tube of the boiler to pass into a vessel kept cool by being surrounded with cold water. Thi action of water under heat is a most usefn illustration of tbe changes of state caused by beat. Exp 24. Starting for instance with watei as a solid in the shape of ice, we can, by increasing the heat applied, canse it to pasi first into tbe liquid state, for which state wi commonly reserve the name water, and ther with farther heat, into the state of a gas, in which state we term it steam. It is ti be remembered that ice, water, and stean are identical as far as chemical compositioi is concerned. Tbis experiment shews us tbai the state in which we may find many sub stances is dependent simply ou the present temperature of the earth. If for instance thi temperature of the earth were greatly lowerei then water would be not a liquid but i crystalline solid or rock, or if the heat of thi earth were greatly increased, then all thi waters of the globe would, as gaseous steam form part of the atmosphere, as indeed it if generally supposed they at one time did. Exp 25. The action of heat on pbospborm shews that beat sometimes quickens ohemica' combinations that wonld go on slowly at ordinary temperatures. Phosphorus is s substance that readily combines with oxygen, and even at ordinary temperatures tbis combination goes on if phosphorus be exposed ti tbe air, but subject the phosphorus to a litth heat from a spirit lamp, and note bow much tbe process is quickened. With heat the combination of phosphorus with oxygen, or as it is usually termed, tbe burning or combustion of phosphorus, is almost instantaneous. (To be Continued).