We have received a letter from a correspondent in Boston, containing an article from the “ Haverhill Gazette,“ Mass., on the above subject. The author of it is evidently well acquainted with his subject, and it is one of much importance to the community. The article in question says, “ I have made a full investigation of the chemical character of the various liquids sold by dealers for the purposes of artificial illumination, and have subjected these compounds, and the lamps designed to be used with them, to very accurate experiments. Dangerous frauds have been continued tor years by unprincipled men in the sale of those compounds without exposure,“ He asserts that a mixture of turpentine and alcohol, colored with turmeric, has been sold by dealers for years, under the name of “ vegetable oil,“ with the unblushing assertion that it was perfectly safe and unex-plosive. This mixture afforded by the distller at 50 cents per gallon, at once in the hands of an unscrupulous dealer advances from SO cents to 70 cents per gallon, by adding one cent's worth of turmeric to it, and changes from a volatile dangerous hydrocarbon or burning fluid to the safe vegetable oil. Such are some of the tricks of trade. Every case of this kind should be punished with severity. The author (we do not know him,) of the article in question, states that Newell's wire gauze lamp, which has been noticed in the Scientific American, is but a modification of the one patented by Isaiah Jennings, of this city, N. Y., in 1836, and the question is asked of us, if this is true, as Newell's has been sold for a patent lamp. We are not aware of any patent har- ing been granted for it, and we cannot disco- ver that one was granted to I. Jennings in 1836, but there was one in 1841, which combined a cotton percolator and wire between the fluid chamber and the flame. All volatile hydrocarbons are explosive, that is, any fluid employed for giving light, if it evaporates at a low heat, and this vapor is suffered to mix with the atmosphere it becomes an explosive gas. None of what are called the explosive fluids will explode until they become vaporized, it is the vapor, not the fluid, that is the cause of explosions. The author of the article in question asserts that in the lamps of Newell which he saw, there were orifices in the cap, made, as he was informed, at the suggestion of Dr. Jackson, for the purpose of letting off the vapor—a safety valve. If these lamps have small holes in their caps, it is a scientific blunder, for the grand object to prevent lamp explosions is to exclude the air. The pressure of heat Irom the vapor of an apartment, can never be so great as to explode the lamp. The safety of such lamps depends upon excluding the fluid and vapor from the atmosphere. A perfectly tight lamp never yet exploded. As we have stated more than once, we say it again, fluids should never, under any conditions, be used in a house where there are children or servants. In this vicinity there i a dangerous burning fluid sold, by the name of “ Rosic Oil,“ under the pretence that it is a safe unvolatile hydrocarbon. Five minutes before writing this, we examined some of this “ Rosin Oil,“ which the purchaser supposed was something very different from a turpentine mixture: thus people are often deceived by names. There is an oil made from rosin by its destructive distillation, but not a burning fluid. with a similar degree of heat. The action of the acid is not less constant with the oil of poppies ; experiments, moreover, prove that the developement of heat due to this oil is really at 88'4, instead of 71*or 74 degrees, as the direct experiment indicates. This process of analyzing may be applied to the olive oils of commerce; these oils are oiten adulterated only with oil ot poppies, and in such a case their analysis can be made with certainty, if their qualitative composition is sure. But how would it be in case of other oils t In answer to this inquiry, I have fixed the rise of temperature produced by most pure oils, it results irom my researches that the oil of ben and oil of tar furnish almost the same disengagement of heat as olive oil. That the other oils produce a much greater disengagement of heat by means of which they can easily be distinguished from olive oil. Finally, that the drying oils give much more heat than the non-drying oils, and may be easily known. The