which he has akeady given an account in a former communication, gives tabulated results of his chemical examination of several va- rieties of gamboge, and formulae expressing their chemical constitu- tion. A detailed account is given of the properties of the gambodic acid, and of the salts it forms with various bases, such as the gam- bodiates of potash and soda, of ammonia, and of different earths and metals, particularly lime, strontia, magnesia, lead, copper, zinc, and silver. He concludes from this investigation that the most probable formula for gamboge is C^q Og. In the analysis, however, of every specimen, there occun'ed a deficiency of carbon, amounting to nearly one per cent. ; a deficiency supposed to be due to a change produced during the preparation of the natural resin for the market. By a heat of 400° Fahr. gamboge undergoes a partial decomposi- tion; a resin, soluble in alcohol, and another resin, insoluble in that menstruum being formed : the formula representing the latter being C^g ^00, Oy. Gamboge forms mth the metallic oxides nume- rous salts, the existence and constitution of which, however, the ex- periments of the author only render probable.
The inquiries of the author were next directed to the chemical constitution of the resin of guaiacum, and to the properties of the salts it forms with various bases. He then examines the acaroid resin, which exudes from the Xanthorrhiea hastilis, and is often known by the name of Botany-bay resin, or yellow gum ; and finds its formula to be C40 Hgo 0,3, showing that it contains more oxygen than any other resinous substance hitherto analyzed.
The general conclusions drawn by the author from these researches are the following.
1 . Many of the resins may be represented by formulae exhibiting their elementary constitution, and the weight of their equivalents, in which 40 C is a constant quantity.
2. There appear to be groups, in which the equivalents, both of carbon and the hydrogen, are constant, the oxygen only varying ; and others, in which the hydrogen alone varies, the two other ele- ments being constant.
In the third part of the same series of investigations, the author examines the constitution of the resin of Sandarach of commerce, which he finds to consist of three difi'erent kinds of resin, all of which possess acid properties. In like manner he finds that the resin of the Pinus abies, or spruce fir, commonly called Thus, or or- dinary Frankincense, consists of two acid resins ; the one easily so- luble in alcohol, the other sparingly soluble in that menstruum. The gum resin olibanum, of commerce, was found to consist of a mix- ture of at least two gum resins, the resinous ingredient of each of which differs from that of the other in composition and properties.
7. " On the Markings of the Eel-back Dun variety of the Horse, common in Scotland;" in a letter to P. M. Roget, M.D., Sec. R.S. By W. Macdonald, M.D., Fellow of the Royal College of Physicians of Edinburgh, F.R.S. Ed., F.L.S., &c. Communicated by Dr. Roget.
