Having seen nothing on the subject of Hi liochromy, in your columns, since my formi communication, I now present you another the same subject, which will I hope stimulai artists, and those fond of scientific experimen to further discovery ; I propose briefly to di cuss the action of light on the chloridate plate, and to give you the results of a few e; periments on it. It is well known to ch mists that light produces little or no chang on perfectly pure chloride of silver, but thi it is rapidly blackened if organic matter b present, and that this organic matter is gen rally found in the water with which it hi been washed, or in the solution from which: has been precipitated. When the chloridate plate is exposed to light, this organic matte is decomposed, oxygen being eliminated, an the free nascenthydrogenreduces thechlorid to a metallic condition, and an opposite stat of electrical excitement is induced. Now M. Becquerel and NiepcedeSt. Victo have proved that it chloride of silver contain ing a slight trace of copper be exposed to th prismatic spectrum, or to rays of different co lors, while undergoing this reduction, it is sue ceptible of coloration after a protracted expo sure. From this it would seem that this pro cess might be much accelerated, if we wer careful to aid nature in her operations, insteai of trying mere hap hazard experiments, no based on rational theory. I will show by few experiments that this may be done, am to avoid being too prolix, will, at present speak of the chloridated silver plate, unacce lerated by iodine, bromine, fluorine, chrome or their compounds. If the plate, covered with the enamellei chloride of silver prepared by Niepce's pro cess, be exposed to a current ot hydrogei while receiving the image, the process wil be much accelerated, and the image will b impressed in irom halt an hour to an hour; according to the amount of gas passed into thf camera, the light, temperature, electric state of the atmosphere, &c, instead of requiring from three to five hours, as in the original process, and the colors of the picture will be impressed on the plate in all their original beauty. This experiment may be very easily performed, it only requiring a few grains 01 zinc in a small vial, containing dilute sulphuric acid. The vial and its contents may be placed in the camera, and the hydrogen being nascent is in its most active state, and as it is perfectly transparent, it permits the light tc act on the plate, while it is itself engaged in reducing the chloride, which it is only capable of doing in sunlight. The hydrogen, probably from its affinity for oxygen, hastens the decomposition of the organic matter, and assists in reducing the chloride, thus acting as a deoxydating and dechlo-ridating agent. There is, however, sufficient hydrogen contained in the combined organic matter, to effect the reduction of the chloride, hence it is probable that the excess meraly hastens the decomposition. Following this train of investigation, I have tried many other reducing agents both liquid and gaseous. The most important liquid agents tried have been, the proto sulphate and nitrate of iron, ferocyanide of potassium, pro-tochloride of tin, and the fluorides of potassium and sodium. The principal gaseous agents tried are hydrogen alone and in combination with carbon and sulphur, ammonia, sulphuric ether in vapor, chloroform vapor, sulphuret ot carbon, chloride of sulphur, hy-dro-sulphuret of ammonia, and sulphurous acid. As very remarkable results followed from the application of the gases, I will speak of them more particularly. Sulphurous acid has a strong tendency to abstract oxygen from organic bodies, it also unites with chlorine in sunlight, and so do light and heavy carburet-ted hydrogen, the latter, indeed, without the influence ot light. Sulphurous acid abstracts oxygen from organic bodies, with which it combines, lorming sulphuric acid, and sulphuric acid renders chloride of silver unchange-'e 6 light by destroying the organic matter with which it is combined. I hence inferred that it might be used tor the double purpose of reducing and fixing the pictured "That it i a powerful accelerator is certain, the fixin( requires iurther experiment. Pictures ma; be obtained with this gas in hah an hour, b] passing it nascent and in sufficient quantity ii the camera and the colors are preserved There is, however, sometimes a little sulphu: deposited under the enamel, which gives thi light parts of the picture a yellowish cast This color may sometimes be removed b] heating the plate. Carburetted hydrogen act; still quicker, probably trom the free carboi which results from its decomposition being ! powerfm reducing agent, and as the carbor is not left under the enamel it probably passei off under the form of the volatile chloride carbon. I obtained one picture in five minutes, by passing into the camera the gases generated from the distilling alcohol and sulphuric acid in a retort. The gases formed were olefiint gas and sulphurous acid, mixec with a little light carburetted hydrogen anc sulphuric ether. The colors were very tairlj represented, but not as good as I had previously obtained ; I considered this experiment as very encouraging, but having only lately tiied it, have not repeated it by itself without the agency of electricity. , As electricity is a powerful agent in decomposing chemical compounds, it might be naturally inferred that it would aid in this process. 1 have often tried it but without, until lately, any very important results. Dry chloride o! silver is not decomposed by electricity, yet its decomposition by light, and other agents, may. by it. be much accelerated, and I did not at firfet use a sufficiently powerful current. J now render the plate a part of the conducting medium which terminates at the positive pole, and terminate the poles in water, to which some saline constitutent has been added, and by the decomposition of the water am enabled to judge of the power ot the current. By using the gases at the same time that the plate is thus excited, I have been enabled to take pictures in trom four to five minutes, which would otherwise require from three to five hours for their production. These pictures are developed under a hard, tough enamel of chloride of silver, cannot be rubbed out by the fingers, and will even bear considerable buffing, and, if the enamel is thick, are improved by the operation. 1 have not been able to permanently fix the picture, but it will keep a long time, if not exposed too often and too long, to the light. From the above experiments it seems that a prolonged exposure is not necessary to produce coloration, hence agents of great energy may be employed in reducing the chloride. That coloration may be produced, it is important, I think, that the picture by whatever process it is taken, be positive, and complete on its removal from the camera. For fixing, it is important that all the organic matter be destroyed, and then, I believe, it will be fixed. I am at present engaged in experimenting with iodine, bromine, fluorine, sulphur, chrome, and copper, and their compounds, deposited on the silver plate by electric action, or otherwise, but have not, as yet, any results sufficiently matured to publish, though I have produced coloration. Great care is requisite in preparing the enamelled plate of chloride, and some experience is required to judge at what state of its preparation it is most sensitive to light, yet any artist can after a few experiments prepare it. I have had but little time for experiment, owing to the pressure of other duties, and the weather here has been lor the last few weeks unfavorable. I am not a daguerrean artist, and am under many obligations to Messrs. Bisbee and Robinson, of this city, for the loan of a camera and other apparatus for my experiments. Having been obliged also to make the greater part of the chemicals used, 1 have as yet, been able to make, but a very meagre investigation of this interesting subject Jas. Campbell. Dayton, Ohio, Jan. 20, 1853. [The above communication from the pen of Mr. Campbell is the most important that has ever been published on the subject of " Heliochromy " in this or any other country We advise all our readers who feel an inte rest in " sun-coloring," to read the article ith attentionEd.