MessRs. Editors:A Polish gentleman once told me that a liquid salt (pcrhaps fluid borate of soda) was sold in Poland, which could be used with a brush and was employed over the whole of the inside of rooms, and rendered them completely fire-proof, in place of alum water, or solution of iron or tin. There appears to be, a want of some article to fasten manuscripts in place of vulcanized india-rubber, which I have found to perish soon. Could not a strap of Chamois leather be easily contrived, say half an inch wide, attached to one side, and passed through an eye, similar to the clastic or French gloves? Your article, " Steam Power versus Wind," reminded me of a conversation I had with a person in Tarrytown, who remarked, he had a freighting vessel there which made its passage almost equal to steamboats. On enquiring about her construction, I found she was built somewhat of a seow shape, drawing little water, in fact like the ice boat, working upon the surface and not displacing mueh water, and kept to the wind by an ingenious center board, which the helmsman can raise or depross at pleasure. I think you are mistaken about an artificial ultramarine being produced from cobalt. The cost of the cobalt blues is more than ten times that of the average of artificial ultramarine, so much so that great pains have been taken to rid the same of the purplish hue which it has, so as to make it resemble cobalt, which, when pure, is the only pure blue color known among artists, and is much more costly for glass and porcelain pigments than any other, as I know, selling them both to consumers. If marble is simply a carbonate of lime, why cannot it be imitated somewhat like the plaster of Paris or sulphate of limo, and in place of tedious sculpturing, why cannot cuts be made, like those of bronze? S. N. Dodge. [Our correspondent's letter is full of varied information ; therefore, we have pleasure in adding to it a few remarks of our own. The liquid glass sold in Poland is soluble silicate of soda ; it is much used on the continent of Europe, and might with advantage be employed here. There is a variety of ultramarine made from cobalt combined with alumina, but the best is manufactured from alumina, silica and soda, with a little sulphur, in fact it is the artificial production of the mineral lapis lazuli. The reason why marble cannot be very S11Ccessfully imitated, is that its beauty depends upon the slowness with which it has bcen deposited and the pressure to which it has becn subjected ; we must attain some mechanical equivalents for these forces, or we shall never be able to compete with the rocks of Mother Nature, and, as yet, we do not possess them.