Mr. T. Webster on the Colour qfStemn. 185 



version must be much more rapid for high than for low steam, 

 and consequent!}- the stages of condensation must be passed 

 more rapidly in the former than in the latter case. I am not 

 aware of the existence of any authentic observations on the 

 temperature of highly elastic steam when expanded into a 

 partial vacuum, or at different distances from the orifice, nor 

 do I think that the thermometer will furnish the requisite 

 knowledge, as its changes are slow, and the changes to be as- 

 certained are exceedingly rapid, and unless noted at the in- 

 stant the numerous sources of inaccuracy will embarrass the 

 results. However, the thermometer will furnish no evidence 

 respecting the caloric of elasticity, as it has been termed, or 

 of the state of the particles in the progress of the steam to- 

 wards condensation. If these successions of colour are due 

 to the stage of condensation, they ought, on the preceding 

 principles, to succeed each other more rapidly, and the cri- 

 tical stage ought to appear nearer the orifice the higher the 

 elastic force of the steam. Thus I conceive your observations 

 on the colour of steam may be employed as a test of the truth 

 of the above principles, as a measure of the conversion of sen- 

 sible into latent caloric when steam suddenly expands, as a 

 means of obtaining some distinct and accurate knowledge of 

 the elasticity and temperature of steam during its expansion — 

 laws which cannot be ascertained by the mercurial column 

 and the thermometer, owing to the time which their indica- 

 tions require, and finally of the various transition states through 

 which the particles pass betwixt a colourless elastic and in- 

 elastic fluid. 



Your experiments and observations, and the conclusions 

 which may be derived from them, are the only ones on which 

 I can rely with confidence, as supporting an opinion long en- 

 tertained by me respecting the theory of clouds. I have never 

 been able to assent to the theories generally circulated on this 

 subject, but have considered clouds as masses of vapour still 

 preserving its gaseous form, but in a different stage as regards 

 condensation from the surrounding vapour, and that while 

 their shape depends on the manner in which the transfer of 

 heat is going on, their colour depends on the state of transi- 

 tion in which the particles of the mass exist, and on the jwsi- 

 tion of the mass with respect to the illuminating body and the 

 spectator. Thus their appearance will continue the same only 

 so long as the above conditions are unaltered, and a cloud 

 which appears stationary or in motion may in reality be a 

 mass of vapour in motion or at rest. 



Should these brief remarks be of any value in themselves, 

 or turn the attention of anv one to the important subject which 



