Retrospective Criticism. 215 



that there is an actual and visible ascent of the sap in all healthy plants, while in growth, is also 

 certain, as well as an issue of it in all directions from wounds, whether on the roots, stem, or on 

 the branches. 



There is no doubt that the knowledge of this branch of physics has been obscured by both 

 ancient and modern terminology. The young man, whose mind is occupied with notions of the 

 circulation of the sap ; its ascent in spring, and its descent in the autumn ; its simple state as a 

 pure liquid, and its matured quality as cambium ; its organisable properties, &c. &c, will be sadly 

 perplexed while he endeavours to reconcile the doctrines he has been taught, with the phenomena 

 before his eyes. To account for the direct, lateral, and retrograde motions of the sap, he must 

 call to his assistance almost all the powers of nature, before he can form any thing like a rational 

 idea of one of her most common processes. 



A clearer view remains to be opened to the young botanical physiologist. No individual has a 

 better opportunity to enter on such a scrutiny than Mr. Thompson himself; and it is to be hoped 

 his brethren will hear from him again, and that he will excuse the liberty I have taken in what I 

 have offered to his notice. — I am, Sir, &c. J. Main. Chelsea, July 10. 1829. 



Qualities of Composts and Soils. — Sir, On the very sensible remarks of your 

 correspondent W. R. Y. of Sheffield, p. 404. Vol. II., and your own observations accompanying 

 his communication relative to the qualities of composts and soils, 1 am induced to hand you a 

 few remarks : — 



That there is great variety in soils; that they differ in respect of constitution and qualities; 

 and that plants are variously affected by such constitution and qualities, are universally evident 

 and well known. Their luxuriance and perfection depend entirely on the suitableness of the 

 station : and not only are their characteristic properties liable to be changed, but even, as has 

 been proved by your correspondent, the colour of their flowers. Hence the necessity of some 

 test, furnished by the science of chemistry, which would enable practitioners to ascertain as easily 

 as possible the inherent noxious qualities of soils or composts, together with instructions for ap- 

 plying counteracting properties in the preparation for a crop. This, notwithstanding all that has 

 been written by Sir H. Davy and his chemical brethren in France, is still involved in technical 

 obscurity, and consequently out of the reach of plain practical men : so that, as you very properly 

 advise, it would be well worth the attention of some chemist of leisure to collect what is already 

 known, and by experiment confirm or dissipate what only rests on suspicion, in order that some 

 brief manual may be published for general information. 



That the colour of the hydrangea is mutable has been long known, and attributed to the 

 qualities of a certain kind of loam found somewhere in the neighbourhood of London ; but lam 

 not aware that the peculiar essence has been detected, as it appears to be by W R. Y. On the 

 subject of the accidental change of the tints of flowers, M. Decandolle (Hort. Sue. Trans.) 

 alleges that " blue and yellow are the fundamental types of the colours of flowers, and that they 

 mutually exclude each other. Yellows pass readily into red or white, but never into blue ; and 

 blues are changed into red or white, but never into yellow." In the case before us, we observe 

 pink changed into purple by the agency of " the oxide of iron." This fact is curious, and 

 interesting to the practical florist; especially if assisted by chemistry in the application of gaseous 

 or other qualities. It is said that the Chinese florists are acquainted with the secret of changing 

 the tints of flowers, and particularly those of the Pc&bnia Moulan, by the addition of certain qua- 

 lities to the soil. 



As to the aeration of soils, it isincontestible that they are deteriorated by unnecessary exposure 

 to the sun and air. That precious pabulum, or vegetable food, in maiden earth, or even in com- 

 posts collected and amalgamated with the greatest care, may, by repeated turnings, be com- 

 pletely exhausted, and much sooner than by the heaviest crop! I cannot, as a chemist, describe 

 what those peculiar qualities are ; but, as a gardener, I am certain that the best and most nutri- 

 tious element in soils is fugitive, and, whether aqueous or gaseous, flies off in exhalation : and, 

 as a farmer, I know well that one light, thin, straggling crop, which too freely admits the parch- 

 ing rays of the sun throughout a whole summer, will certainly be succeeded by another thin 

 crop ; that a clover ley, which has been mowed twice, is in better order for wheat, than the same 

 would be if constantly divested of its herbage by cattle ; and that the ground under a heap ot 

 stones, which has lain in a fallow field throughout a summer, will, by bearing superiorly, be 

 visible for the two succeeding crops ! So much does the shading of ground preserve its nutritious 

 qualities. Salts, in their concrete or crystalised state, perhaps may remain undiminished in a 

 parched soil, but other substances certainly fly off. 



But it is necessary to add, that all soils whatever require occasional fallowing, for the purposes 

 of cleaning from weeds, and pulverisation for the reception of seeds. The summer sun and 

 drought are necessary for the first; and both the summer heat and winter frost are absolutely 

 necessary (on heavy clayey soils) for the last. Such practice is sufficiently obvious ; but let it not 

 be imagined that the rays of the sun in summer, or the want of them in winter, can ever add one 

 particle of vegetable pabulum to the staple of the ground. 



Spade-ridging wet or clayey land, for the purpose of drying or pulverisation in gardens, and 

 clasp-bouting or back-bouting with the plough in fields, are both judicious practices ; but 

 ridging sandy or light soils can do no good any where. 



Connected with digging and ploughing, trenching the surface of the ground is one of the most 

 effective operations for securing the prosperity of trees, and indeed of any crop. It not only gives 

 scope for the extension of the deep-rooting plants, complete pulverisation, and admixture of the 

 superstratum, but also for an effect which very often escapes notice ; namely, the facility it gives 

 for the ascent of subterranean heat and dew ! ! ! I have used these marks of admiration merely 

 to save you the trouble of placing them ; because it is more than probable you will think the 

 expression, if not the idea, a wild one. 



But let us consider : — It cannot be denied that there is an inexhaustible fountain of both heat 

 and moisture in the bowels of the earth ; miners, well-diggers, all attest this. The heat of main 

 spring water is never, summer or winter, at a lower degree of temperature per Fahrenheit than 

 40°. According to the depth, the heat increases ; so that water drawn from the depth of 300 ft. 

 yields a temperature of £6°, and in some instances even a higher temperament. From this grand 

 source of heat and humidity there is constantly issuing forth a united stream of these two ele- 

 ments, in the shape of vapour. Look at the mouth of a well, of a coal-pit, or other mine ; 

 observe the steam from the door of an ice-house, cellar, or from any deep excavation in the earth ; 

 and last, not least, see how much more copious this vapour is from loose or new-trenched ground, 

 than appears, or can be condensed, on the firm and indurated surrounding surface. If light pre- 

 vents seeing this, place a well-glazed hand or bell glass on the trenched ground, and another on 

 a gravel walk or road, and then see which receives and condenses the greater share of this 



p 4 



