or upon the surface of solid bodies, their repulsion ceases, and their whole chemical action is exerted.  Thus combinations which oxygen cannot enter into, decompositions which it cannot effect while in the state of gas, take place with the greatest facility in the pores of platinum containing condensed oxygen.  When a jet of hydrogen gas, for instance, is thrown upon spongy platinum, it combines with the oxygen condensed in the interior of the mass; at their point of contact water is formed, and as the immediate consequence heat is evolved; the platinum becomes red hot and the gas is inflamed.  If we interrupt the current of the gas, the pores of the platinum become instantaneously filled again with oxygen; and the same phenomenon can be repeated a second time, and so on interminably.

In finely pulverised platinum, and even in spongy platinum, we therefore possess a perpetuum mobile—­a mechanism like a watch which runs out and winds itself up—­a force which is never exhausted—­competent to produce effects of the most powerful kind, and self-renewed ad infinitum.

Many phenomena, formerly inexplicable, are satisfactorily explained by these recently discovered properties of porous bodies.  The metamorphosis of alcohol into acetic acid, by the process known as the quick vinegar manufacture, depends upon principles, at a knowledge of which we have arrived by a careful study of these properties.

LETTER III

My dear Sir,

The manufacture of soda from common culinary salt, may be regarded as the foundation of all our modern improvements in the domestic arts; and we may take it as affording an excellent illustration of the dependence of the various branches of human industry and commerce upon each other, and their relation to chemistry.

Soda has been used from time immemorial in the manufacture of soap and glass, two chemical productions which employ and keep in circulation an immense amount of capital.  The quantity of soap consumed by a nation would be no inaccurate measure whereby to estimate its wealth and civilisation.  Of two countries, with an equal amount of population, the wealthiest and most highly civilised will consume the greatest weight of soap.  This consumption does not subserve sensual gratification, nor depend upon fashion, but upon the feeling of the beauty, comfort, and welfare, attendant upon cleanliness; and a regard to this feeling is coincident with wealth and civilisation.  The rich in the middle ages concealed a want of cleanliness in their clothes and persons under a profusion of costly scents and essences, whilst they were more luxurious in eating and drinking, in apparel and horses.  With us a want of cleanliness is equivalent to insupportable misery and misfortune.