No. IL] ON SUDDEN ALTERNATIONS OP TEMPERATURE. 13 



he has arrived are deduced from the appearance presented by the plant after it 

 was tkavjed. He found the tissue more or less lacerated, the contents of the air 

 and sap vessels intermingled, and the colouring matter and other secretions de- 

 composed. He attributes the laceration to the freezing and consequent expan- 

 sion of the juices, but this cannot be the necessary consequence of that freezing, 

 since it does not appear, if the whole tuber or leaf be slowly thawed, I wouH 

 explain the phenomena as follows: — 



1°. When the leaf, fruit, or tuber freezes, the fluid portions slightly expand 

 in becoming solid, but the air in the air vessels contracts in at least an equal de- 

 gree, and thus allows a lateral expansion of the sap vessels sufficient to prevent 

 lesion. When the temperature is slighdy raised, the air expands but shghtly, 

 and ice is melted long before the gaseous substances reach their original bulk. 



2°. But if the rays of the sun strike suddenly upon the leaf or fruit, the sur- 

 face may at once be raised in temperature 30° or 40° F. The air will conse- 

 quently'expand suddenly, and before the s.ip is thawed may have distended and 

 torn the vessels, and caused sap and air to be mutually intermingled. 



3°. But the moment the sun's rays strike upon the green leaf, its chemical 

 functions commence. It begins to absorb and decompose carbonic acid : and 

 as in the frozen part of ihe leaf the circulation is not, and in consequence of the 

 lesion cannot be, established, the chemical action of the sun's rays must be ex- 

 pended upon the stagnant sap ; and hence those changes not only in the sap 

 Itself, but even in the solid parts, which are seen to take place in the withered 

 leaf 



4°. Though not in a state of growth, the tuber of the potatoe contains the 

 living principle, and there must be such a circulation going on in its interior as 

 to mamtain an approximate equilibrium of temperature throughout its sub- 

 stance. A sudden thawing of the exterior, will, as in the leaf, expand the air 

 before the circulation can be established throughout the frozen mass. The solid, 

 fluid, and aeriform substances which nature has separated and set apart from 

 each other, will thus all be intermingled, and from their mutual action, those 

 chemical changes of which we know the starch of the potatoe to be susceptible, 

 will speedily ensue ; — in other words, the potatoe will rot. 



The practical applications of these views are numerous. If a sudden frost 

 come on, — protect your delicate flowers in the early morning from the rays ot 

 the approaching sun, and cover with straw or earth the potatoes which have 



No. III. 



RESULTS OF EXPERIMENTS ON PRACTICAL AGRICULTURE DURING 

 THE SPRING AND SUMMER OF 1841. 



{See Appendix, No. 1., and Lectures VIII. and IX.) 



In a previous article inserted in this Appendix, and which was published 

 early in the present spring (April, 1841,) I ventured to offer to the practical ag- 

 riculturist some suggestions in regard to the experimental use of certain un- 

 mixed manures. From the results of these experiments, which I was quite sure 

 some of the many zealous agriculturists of the day would be induced to under- 

 take after the manner, and with the precautions, 1 had pointed out, I anticipated 



