When Daguerre first published his great discovery, the European public regarded his metal tablets with feelings of wonder: we have grown accustomed to the beautiful phenomena of this art, and we have become acquainted with a number of no less beautiful processes on paper, all of which, if studied aright, must convince the most superficial thinker, that a world of wonder lies a little beyond our knowledge, but within the reach of industrious and patient research. Photography is the name by which the art of sun-painting will be for ever known. We regard this as unfortunate, conveying as it does a false idea,--the pictures not being _light-drawn_. Could we adopt the name given by Niepce to the process, the difficulty would be avoided, since Heliography involves no hypothesis, and strictly tells the undeniable truth, that our pictures are _sun-drawn_. That pictures can be produced by the rays from artificial sources, presents no objection to this; these rays were still originally derived from the sun. By whatever name we determine to convey our ideas of these phenomena, it is certain that they involve a series of effects which are of the highest interest to every lover of nature, and of the utmost importance to the artist and the amateur. By easy manipulation we are now enabled to give permanence to the charming pictures which are produced by means of that pleasing invention of Baptista Porta, the _Camera Obscura_. Any image, which being refracted by the lens of this instrument falls upon the table in its dark chamber, may be secured with its most delicate gradations of shadows, upon either a metallic or a paper tablet. But let us proceed to the examination of a few of the more striking phenomena of these chemical changes. To commence with some of the more simple but no less important results. Chlorine and hydrogen will not unite in darkness, nor will chlorine and carbonic oxide; but, if either of those gaseous mixtures is exposed to sunshine, they combine rapidly, and often with explosion. A solution of the sulphate of iron in ordinary water may be preserved for a long time in the dark without undergoing any change; expose it to the sunshine, and a precipitation of oxide of iron is very rapidly produced. The mineral chameleon, the _manganesiate of potash_ in solution, is almost instantly decomposed in daylight; but it is a long time before it undergoes any change in darkness. The same thing occurs with a combination of platinum and lime: indeed, it appears that precipitation is at all times, and under all circumstances, accelerated by the solar rays. As these precipitations are in exact agreement with the quantity of actinic radiation to which the solutions have been exposed, we may actually weigh off the relative quantities, representing in grains the equivalent numbers to the amount of actinism which has influenced the chemical compound.[119]