Water of Kespiratiox. 429 



experiments, shows that as a rule the relative humidity of the air inside the 

 chamber becomes lower and lower as the fast progresses. The percentage 

 relative humidity is determined in large measure by the rate of ventilation of 

 the chamber. The more rapid the rate of ventilation, the greater the amount 

 of air withdrawn and completely deprived of its moisture by passing through 

 concentrated sulphuric acid, and hence the larger the volume of dry air 

 returned to the chamber. While in the 2-hour periods it is more than probable 

 that the rate of speed of the electric motor and consequently the volume of air 

 passing through the chamber may vary considerably, it is highly improbable 

 that in the course of 24 hours any marked variations in the amounts of air 

 passing through the chamber are to be observed. In the earlier series of 

 experiments made with the apparatus in a very different form, a mechanical 

 air pump was used and a large number of records were obtained to show the 

 total ventilation per 24 hours. Inasmuch as the same electric motor, shafting, 

 fittings, etc., aside from substituting a rotary blower for the mechanical air 

 pump, are now used, the conditions are not different now than they were 

 before. At that time all the air passing through the chamber was first caused 

 to pass through a large gas meter and thus a record of the total ventilation was 

 obtained. The total ventilation for a large number of days rarely altered more 

 than 3 or 4 per cent from day to day. 106 



It seems probable that the rate of ventilation stays reasonably constant. 

 Under these circumstances, then, any fall in the relative humidity must be 

 ascribed to a diminished loss of water from the body through the lungs and 

 skin and the figures given in table 218 show that this is actually the case. 

 Since, then, there is a diminishing loss of water of respiration and perspiration 

 from the body, even with a markedly diminished relative humidity, it seems 

 clear that at least in experiments with fasting men some factor other than 

 relative humidity determines the loss of water of respiration and perspiration. 



Influence of muscular activity. The only remaining factor that seems in 

 any way connected with the formation of water of respiration and perspiration 

 is muscular activity. The frequent observations that excessive muscular 

 activity results in profuse sensible perspiration justify the assumption that 

 even minor muscular activity would cause an increase in the insensible perspi- 

 ration over that commonly occurring during rest. It might be expected, 

 furthermore, that the muscular activity would determine to a considerable 

 extent the degree of insensible as well as sensible perspiration. 



The muscular activity in experiments of this nature may be measured 

 relatively by three methods: (1) The record of the muscular movements of 

 the subject obtained from the diary, notes of assistants, etc.; (2) the carbon 

 dioxide production, which is an approximate index of muscular activity; and 



106 See table 120, pp. 273-300, U. S. Dept. Agr., Office of Expt. Sta. Bui. 136, in 

 which the ventilation per 24 hours is given for experiments from 35 to 55 inclusive. 



