570 



ment commences. These observations also apply to experiments upon 

 voluntary respiration, where the force, depth, and rapidity exceed that 

 at rest. A space of 1000 to 1500 superficial inches of absorbing sur- 

 face will be necessary with exertion, whilst 700 inches is sufficient at 

 rest. 



There is a variation in the quantity of carbonic acid expired on 

 different days of the week, so that there is an increase after a day of 

 rest. There is also a variation with the season of the year, so that 

 the evolution is the greatest in the spring, then in the winter, and 

 then in the autumn, and it is the least at the end of summer. It 

 increases with cold, and decreases with heat. Hence the quantities 

 evolved at various seasons cannot be compared, neither indeed those 

 in a short period of days, if there have been any considerable 

 changes of weather or habits. The only mode by which the rate of 

 evolution in different days and in different seasons may be compared 

 is by making observations in the morning before food has been taken, 

 and in absolute rest, so as to isolate the effects of season and meteo- 

 rological phenomena from every other influence except the small effect 

 of the conditions of the previous day. This method is almost with- 

 out fallacy. 



URINARY WATER AND UREA*. 



The urinary water should be collected in tall and narrow glasses 

 which are graduated to y^th of an ounce, and covered to prevent 

 evaporation. When travelling, y^th of the quantity emitted at a time 

 may be reserved, and the larger portion thrown away. The glasses 

 must be used during defsecation ; and hence such inquiry cannot be 

 accurately made in women. The various quantities must be retained 

 and collected to the exact termination of each 24 hours ; and after 

 they have been mixed and reduced to the temperature of the air, 

 a sample should be taken for analysis. As the urine decomposes 

 readily in warm weather and when the specific gravity is very low, 

 the analysis must not be deferred later than two days ; but in the 

 opposite conditions four days may elapse. 



There are very great and rapid variations in the quantity of urine 

 evolved ; so that large and small quantities may alternate daily, or a 

 sequence of increase or decrease may be established, or one may fol- 

 low the other for many days. Hence a correct daily average can 

 * See Phil. Trans. 1861. 



