Wcallicr, U'alfr ami Disease. 9 



by the combined action of the luminous, calorific and chemical 

 rays of the sun, and is not, therefore, exclusively due to 

 the chemical ones. Neither is growth always suspended 

 during the niglit, for it is stated that some ])lants accumulate 

 reserve material, which enables them to continue growth in 

 some of their parts that Ijcgin in daylight. Add to this state- 

 ment all of the instances of the known effect of the chemical 

 rays of light in producing chemical combinations of an 

 inorganic kind, and the instances of disassociations of com- 

 pounds, sometimes with explosive force, and we may well 

 be convinced that such a power will not stop short of the 

 vegetable kingdom. It will exercise some of its force, at 

 least, in the molecular action going on at the exposed sur- 

 faces of the human body. The changes in the pigment mat- 

 ter are an instance of it. The withdrawal of the rays of the 

 sun is indirectly instrumental in modifying other elements of 

 our environment, as the humidity and temperature of air and 

 soil. The damp, cool air of the night, the heavy deposition 

 of moisture from a warm air, on the cooler ground, or the 

 rapid radiation from the soil, which takes place into a dry air, 

 will not unfrequently furnish conditions sufficient to disturb 

 the equilibrium of health in an unstable constitution. The 

 latter combination is what brings about largely the extreme 

 variations found in mountain climates between mid- day and 

 mid-night, and again.st which unhealthy persons must care- 

 fully guard themselves. 



The depressing effect of long-continued cloudy weather is 

 to /be mentioned. Few escape it entirely. The contrasted 

 effect of succeeding sunshine is one of our delightful experi- 

 ences, and operating through our nervous system influences 

 favorably our bodily condition. 



These statements are proof of what we said of the benefi- 

 cence of this element of our surroundings; we do not accept 

 our opportunities. We perversely shut out the light from our 

 houses, and in doing so often corrupt the air within them. 

 We bleach our faces and those of our children, instead of sun- 

 painting them. It is said darkness is dirt, and dirt is disease, 

 or at lea.st its ne.sting place. In proof of this we may men- 

 tion the effect of light on various kinds of microbes. It has 

 been investigated experimentally by English, French and 



