20 



A DISCOURSE 



ROOK I. prepared for the vernal, by being barrelled or potted up in moist sand, or 

 earth, stratum super stratum, during the winter ; at the expiration 

 Avhereof you will find them sprouted ; and being committed to the earth, 

 with a tender hand, as apt to take as if they had been sown with the 

 most early, nay, with great advantage : By this means too, they have 

 escaped the vermine, which are prodigious devourers of winter sowing. 



" During the heat of a summer's day, all plants perspire freely from the pores of their 

 " leaves and bark. At that time the juices are highly rarefied. The diameters of the 

 " Trachea?, or air-vessels, are enlarged, so as to press upon and straiten the vessels that 

 " carry the sap. In consequence of which their juices, not being able to escape by the 

 " roots, are pressed upward, where there is the least resistance, and perspire oft' the 

 " excrementitious parts by the leaves and top-branches, in the form of vapour. When 

 " the solar heat declines, the tracheae are contracted. The sap vessels are enlarged, and 

 " the sap sinks down in the manner of the spirits of a thermometer. In consequence of 

 " this change, the capillary vessels of the leaves and top-branches become empty : Being 

 " surrounded with the humid vapours of the evening, they fill themselves from the known 

 " laws of attraction, and send down the new acquired juices to be mixed with those that 

 " are more elaborated. As soon as the sun has altered the temperature of the air, the 

 " tracheae become again distended, and the sap vessels are straitened. The same cause 

 " always produces the same effect ; and this alternate ascent and descent, through the 

 " same system of vessels, continues as long as the plant survives." Georg. Essays, p. 79. 



" Air is to be found in every portion of earth ; as it always contains a solution of 

 " the volatile parts of animal and vegetable substances, we should be careful to keep our 

 " stiff" soils as open as possible to its influence. It passes, both in its active and fixed state, 

 " into the absorbent vessels of the root, and, mixing with the juices of the plant, circulates 

 " through every part. Dr. Hales, in his statical experiments upon the vine, discovered 

 " it ascending with the sap in the bleeding season." 7bid. p. 85. 



These extracts, published in I769 and 1770, will abundantly show that at those early 

 periods I was acquainted with the economy of nature in the consumption of the putrid 

 and noxious particles of the atmosphere, by the vegetable creation. It did not belong to 

 my argument to say, that in consequence of this removal of phhgistoii, the air became 

 " pure and fit for animal respiration." That harmonious reflection arose from the subject, 

 and naturally invited experimental inquiry. And happy it is for the philosophical world, 

 that the prosecution of this inquiry has fallen to the lot of Dr. Priestley, whose experi- 

 ments are so ingeniously conducted, and whose conclusions are so fairly drawn, that I 

 cannot avoid extracting from them so much as regards the present argument. "When 

 " air has been freshly and strongly tainted with putrefaction, so as to smell through the 

 " water, sprigs of mint have presently died, upon being put into it, their leaves turning 

 "black ; but if they do not die presently, they thrive in a most .surprising manner. In no 

 " other circumstances have I ever seen vegetation so vigorous as in this kind of air, which 

 " is immediately fatal to animal life. Though these plants have been crowded in jars 



