This is spread upon flax cloth, (Messrs. Richards & Co., Broad Street, are manufacturers,) which weighs 10, 16, and 32 ounces to the square yard, and is about the same cost as the compound, so that the _weight_ is the _truest criterion of cost_, and is so calculated by the manufactory. * * * * Excepting garments every other description of black goods are made from such forms as I have figured, with very little variation according to the requirements of the engineer, and which needs very little judgment or experience to meet. To those who cling to these heavy goods, the formulas I have rendered will enable them to have supplies at a fair competing charge according to quality, and it is their _own fault entirely_ if they persist in _purchasing 100 per cent. of cheap powder_ at the _same_ charge as for “pure caoutchouc” of a third of their density. Your readers must be good enough not to lose the fact of “specific gravity,” and test all the above goods made with pure, unadulterated India-rubber, by their floating on water. Any deviation from this is a proof of some cheap stuff unfairly introduced. It will be just in proportion as they keep themselves well posted up in this matter, that manufacturers will be sly of throwing into their caoutchouc such mad and infamous proportions of cheap adulterations, and drive them (_for they will not volunteer_) to the necessity of manufacturing from the pure gum, and thus place better disposed traders upon a more just footing, and enable the latter to compete (if need be) upon their own terms, or upon recipes supplied by the engineer. Another fact must not be lost sight of, and that is the use of “_light carbonate of magnesia_,” which, mixed with the gum, does not increase its density, as will in some measure be illustrated by the recipe I shall now figure out, merely remarking that a somewhat heavy carbonate was used, as well as in an unroasted, and hence the somewhat increased density. This article is worth the attention of buyers of rubber goods, as in a moderate quantity it reduces the costs without correspondingly injuring the manufacturer. (_Light buffer spring just sinks in water._) Grind together 25 lbs. Clear Java Rubber, 5 lbs. Para Rubber, 10 lbs. Common Magnesia, 25 oz. Pure Sulphur. Costs 19 cts. per pound—Sells for 60 cts. per pound.