-1859] WRITINGS OF JOSEPH HENRY. 207 



Peculiar mechanical effects are sometimes produced by 

 alternations of thawing and freezing, — as for example in 

 the case of water pipes constructed of lead or other malleable 

 metal. To render this plain let us suppose a lead pipe one 

 foot in length to be filled with water, and after being her- 

 metically sealed at each end exposed to a low temperature; 

 the expansion would merely stretch the pipe, the extension 

 not being sufficient to burst it, and no continuation of cold 

 or increase of its intensity would produce any further effect, 

 as this would merely cause the ice to shrink ; neither would 

 thawing and re-freezing produce any effect, since the water 

 would merely return to its original volume, and the ice again 

 expand to the same extent as before; but if the pipe com- 

 municated with a reservoir of water, so that when the thaw- 

 ing took place, the whole space, enlarged by the previous 

 freezing, were again filled with water, a second freezing would 

 produce another enlargement of its internal capacity, and a 

 third thawing and freezing, under the same circumstances, 

 would repeat the process until at length the sides of the tube 

 would give way. 



Effect of cold on plants. — Plants filled with sap and exposed 

 to a low temperature are variously affected, according to the 

 character of the plant, the duration of cold, and the season 

 of the year at which it occurs. A sudden cold will tend to 

 burst the cells. The velocity of the motion of the sap de- 

 pends principally on the amount of evaporation from the 

 leaves and stems, and this diminishes with temperature, all 

 other things being the same; hence there is a certain degree 

 of cold at which the sap ceases to flow, and the functions 

 of the plant are suspended. 



The different parts of the same plant are killed at differ- 

 ent temperatures below 32°; the more succulent and tender 

 growths suffer first, and the woody portion, or that in which 

 the sap is better defended by non-conducting materials, last. 

 A sudden fall of temperature, (even though it be extreme,) if 

 of short duration, may not penetrate to the sap and produce 

 freezing. It would also appear that the sap of different plants 

 congeals at different temperatures, and it is highly probable 



