42 FIRST PRINCIPLES. 



posed that this action of sunlight is due to the fact of its 

 giving rise to the formation of ozone or peroxide of hydro- 

 gen, both of which are germ destroyers. 



INFLUENCE OF ALTITUDE ON HORSES. 



The chief effects of altitude on the atmosphere are to make 

 it lighter, drier, and colder, and to decrease the percentages of 

 carbonic acid gas and dust in it. Dryness and diminution of 

 carbonic acid and dust are conditions which promote the 

 health of horses. The benefit to be obtained from cold pro- 

 duced by altitude, is dependent on the latitude, other things 

 being equal. Thus in India, Sumatra, and Belgium for in- 

 stance, the Spiti ponies, Deli ponies, and Ardenne mountain 

 horses are respectively far stronger for their size than their 

 relatives in the plains. A difference in altitude (say, of 

 7,000 ft.) which would be highly beneficial to equine life in 

 hot countries, would, by decrease of temperature, be equally 

 prejudicial in cold ones. We may therefore take for granted 

 that cold limits the favourable influence of altitude on the 

 atmosphere. In Tibet, at a latitude about the same as that 

 of Syria, the hill ponies, which are very sturdy animals, live 

 and thrive at an altitude up to at least 14,000 ft. Altitude 

 by rarefying the air, within healthy limits, strengthens the 

 lungs by increasing their work, and in this way often pro- 

 duces a good effect in the early stages of human consumption. 



The beneficial influence of altitude in checking the develop- 

 ment of many disease germs is apparently due, as a rule, to 

 cold and diminution of humidity in the atmosphere ; damp 

 heat being generally favourable to their growth. We find 

 that Horse Sickness in South Africa is practically unknown in 

 places which are at least 6,000 ft. high, unless it has been intro- 

 duced by affected stock. At similar altitudes in India, bursatee 

 (a disease characterised by ulcers which somewhat resemble 

 the " Delhi boils " of man) becomes spontaneously cured. 



