M.—AGRICULTURE. 213 
and phosphorus, either too much or too little oil in the diet decreases the 
amount of calcium absorbed from the intestine. So the ratios of the differ- 
ent minerals to each other and to the organic constituents of the food are 
almost as important as their absolute amounts. 
Recent work on the ultra-violet irradiation of animals is of great impor- 
tance in this connection. The beneficial effects of sunlight have been 
recognised for many generations. Within the past few years it has been 
shown that the beneficial effect of sunlight in nutritional diseases such as 
rickets can be obtained from artificially produced ultra-violet rays. 
At the Rowett Institute we have found that when animals are sub- 
jected to the influence of these rays the amounts of calcium and phos- 
phorus absorbed from the intestine tend to be increased, or at least the 
amount excreted in the feces is decreased, even though at the same time 
the amount excreted in the urine may be increased. This result, which has 
an important bearing on mineral metabolism, has also been obtained by 
other workers. It is of interest to note that the influence of irradiation 
on the absorption of minerals from the intestine appears to be greatest 
when the diet is badly balanced. 
The mode of operation of these raysin unknown. It has been suggested 
that they liberate reserves of some unknown substance which has an 
influence on the absorption of minerals. It is just as probable, however, 
that they affect the association and dissociation of ions and colloids in the 
body fluids, and so alter the balance of the free ions. Since the per- 
meability of cell-membranes is controlled by the balance of these ions, 
it seems possible to suggest an explanation, in terms of physical changes 
in known elements, of their influence on the absorption and retention 
of caleium and phosphorus. 
Ultra-violet irradiation is probably of very great practical importance 
in milk production. It has been shown by balance experiments that there 
is difficulty in preventing a loss of calcium from the body of the cow at 
the height of lactation, at least when the cow is fed indoors, and there is 
reason to believe that this loss of calcium is a predisposing cause of some 
of the diseases prevalent among high-milking cows. If has been found 
that irradiation at the height of lactation can decrease the loss of calcium 
_ in goats, or even convert a negative balance into a positive. The practical 
value of irradiation will, however, not be demonstrated until experiments 
have been carried out with large groups of stall-fed cows in winter, to 
determine whether the irradiation will favourably affect milk yield, breed- 
ing capacity and health. 
From what has been said regarding factors which affect the assimilation 
of minerals, it will be readily understood that, before the mineral content 
of a ration can be adjusted by the addition of inorganic salts, to ensure 
that all the minerals will be absorbed from the intestine in the proper 
amounts and proportions, it is necessary to consider all the existing 
dietary factors. In the first place the requirements of the animal to be fed 
must be estimated ; the amounts already present in the ration must be 
‘determined, and then the influence of any proposed addition on the final 
balance of the ration must be considered. Even when all this has been done, 
it is impossible, in the present state of our knowledge, to do more than make 
an empirical adjustment by adding inorganic salts containing elements 
thought to be deficient in the ration. In a few cases we can predict, with 
