Here is a new employment for botanists, to attempt the production of new species of vegetables by scattering the pollen of various plants over various widowed females. And if these remarks should meet with a favourable reception, I shall be the more induced to dedicate what remains of my life to such experiments, which recommend themselves by being at the same time agreeable and useful. I am persuaded by many considerations that those numerous and most valuable varieties of plants which are used for culinary purposes, have been produced in this manner, as the several kinds of cabbages, lettuces, etc.; and I apprehend this is the reason of their not being changed by a difference of soil. Hence I cannot give my assent to the opinion of those who imagine all varieties to have been occasioned by change of soil; for, if this were the case, the plants would return to their original form, if removed again to their original situation. FOOTNOTES: [Footnote 13: From the _Publications of the Linnaean Society_.] XII JOSEPH BLACK 1728-1799 _Joseph Black, born in 1728 at Bordeaux, France, was educated in Belfast and at the University of Glasgow. Before he took his M.D. degree he showed that alkalies were formed, not by their absorbing “phlogiston,” but by their having carbonic acid gas, or “fixed air,” as a component. In 1753 he was appointed lecturer on chemistry at Glasgow, and in 1776 became professor of chemistry at Edinburgh. In 1763 he announced his discovery of latent heat, a principle which has been of great practical value. He died in Edinburgh, December 6, 1799._ THE DISCOVERY OF CARBONIC ACID GAS[14] Hoffman, in one of his observations, gives the history of a powder called _Magnesia Alba_, which has been long used, and esteemed as a mild and tasteless purgative; but the method of preparing it was not generally known before he made it public. It was originally obtained from a liquor called the _Mother of nitre_, which is produced in the following manner: Salt-petre is separated from the brine which first affords it, or from the water with which it is washed out of nitrous earths, by the process commonly used in crystallizing salts. In this process, the brine is gradually diminished, and at length reduced to a small quantity of an unctuous bitter saline liquor, affording no more salt-petre by evaporation, but, if urged with a brisk fire, drying up into a confused mass, which attracts water strongly, and becomes fluid again when exposed to the open air.