148 Col. Francis Hall's Meteorological Observations 



minutes it overspreads every part of the field of view, which 

 by this singular process has been metamorphosed from a 

 space ahnosl entirely dark, into a luminous one, mottled with 

 all manner of colours. Should the observer happen to have 

 quitted his instrument in the mean while, and during his 

 absence this change have taken place spontaneously, he would 

 hardly be able to persuade himself that his adjustments had 

 not been deranged, and some new object placed before the 

 microscope. 



Tins very beautiful phaenomenon no doubt arises from the 

 following cause, viz. that the crystalline state or arrangement 

 of particles which nitre assumes at the temperature at which 

 it first solidifies after fusion, is no longer suitable to it when 

 grown perfectly cold ; so that its condition is then one of un- 

 stable equilibrium which the slightest force is capable of sub- 

 verting. By touching it with a needle a disturbance is pro- 

 duced, which propagates itself from the disturbed point 

 throughout the entire mass. 



But even if it is not touched the same change will take 

 place spontaneously a few minutes later. 



If however we touch it prematurely, as, for instance, during 

 the first minute after it has become solid, this change does 

 not take place. 



We may then trace lines or letters upon the darkened film 

 with the point of a needle, and these lines will appear lumi- 

 nous, in consequence of the crystalline particles which the 

 needle displaces being thrown into such positions as to de- 

 polarize the light. But this does not disturb the rest of the 

 field of view, which remains quiescent for several minutes 

 after, and then changes spontaneously, as I have endeavoured 

 to describe. 



XXIII. Meteorological Observations made during a residence 

 in Colombia between the Years 1820 and 1830. By the 

 late Colonel Francis Hall.* 



¥F the materials of science could be gathered only by the 

 *■ scientific, the following collection of observations would 

 be a useless labour; but it frequently happens that in distant 

 countries the opportunity of observing natural phenomena 

 falls to the lot of those very ill-fitted in most respects to profit 

 by it. The genius of a Humboldt, like an incantation of sci- 

 ence, descends upon the New World but once in a series of 

 ages. The most that can be done by an ordinary observer is 

 to offer his mite, a single stone towards the pyramid of know- 



* Communicated by Prof. William Jameson, of Quito, to Sir W. J. 

 Hooker, F.R.S., and by him to Phil. Mag. 



