On a recent visit to Bangor, in North Wales, I was struck with the nice firm turf which was in the garden; and upon inquiring of the gardener, he informed me that the turf came from the seeds blown from the hills, and that it required great care on the part of the farmers to keep it under, or it would be exceedingly injurious to land and buildings if neglected. When it grows on walls it splits them by the capillary expansion of its roots between the bricks operated upon by damp hot weather. I have seen this lichen destroy the pillars of a gateway three feet thick. Mill-stones are made in Germany out of granite, by means of willow pegs being driven into holes thinly covered with water; this causes the willow to act by capillary expansion, forcing the mill-stones of the required size out of the rock. It is of the utmost importance that the nature of moss and lichen generally should be well studied before constructing sewers, &c., where vegetable matter exists near water. Was it by similar means that the ancient Egyptians and inhabitants of Arabia Petræa cut from the solid rock those vast blocks, in effecting which they do not seem to have availed themselves of any modern mechanical contrivances? The _ferment_, that is, the substances in a state of fermentation and capable of acting on all fermentable bodies, and especially on complex organic compounds, as the blood, exist at all times in the air, but are as a matter of course greatly influenced by a variety of circumstances as regards their effects on man and other animals. It is proved by indubitable evidence that this morbific matter is as capable of entering the system when minute particles of it are diffused in the atmosphere as when it is directly introduced into the blood vessels by a wound. When diffused in the air, these noxious particles are conveyed into the system through the thin and delicate walls of the air-vesicles of the lungs in the act of respiration. The mode in which the air-vesicles are formed and disposed is such as to give to the human lungs an almost incredible extent of absorbing surface, while at every point of this surface there is a vascular tube ready to receive any substance imbibed by it and to carry it at once into the current of the circulation. Thus in certain seasons boils and carbuncles prevail to an alarming extent, and surgeons dare not operate lest they should lose their patients from erysipelas and inflammations, running rapidly into putrescence. In large hospitals the poisonous air in all probability is constantly present, attacking those who have been previously weakened by disease or wounds, or loss of blood; in other words, all those in whom from any circumstance (as by the depression of the vital powers) the complex organic compounds are held loosely together, and are therefore prepared to ferment or to fall into putrescence. NOTE 4.--_Anther._