10 



Scientific Proceedings (117). 



osteoid in the region of the metaphysis and along the shafts of the 

 bones. 



In the paper previously referred to it was shown that the intro- 

 duction of 0.4 per cent, secondary potassium phosphate (K 2 HP0 4 ) 

 in place of an equal weight (replacing about one seventh) of the 

 calcium lactate contained in the rickets-producing diet, completely 

 prevented the development of rachitic lesions; this constitutes 

 an addition of 75 mg. of phosphorus per 100 gm. of the diet. In 

 order to test the counterbalancing effect of phosphate and dark- 

 ness, a series of tests were carried out in the dark with additions of 

 small and increasing amounts of potassium phosphate to the stand- 

 ard dietary (No. 84) ; to one series 25 mg. were added, to another 

 75 mg. (constituting dietary No. 85). 



The rats on these diets were kept in the dark but, to serve 

 as control, half of each series were exposed to sunlight for thirty 

 minutes daily when this was possible. As was to be expected in 

 view of our previous experience and the fact that phosphate tends 

 to protect against rickets, none of the rats which were treated with 

 sunlight developed rachitic lesions. Among the group, however, 

 which were kept at all times in the dark, active rickets developed 

 in spite of an addition of 25 mg. of phosphorus. The addition of 

 75 mg. was found to be sufficient to prevent the development of 

 this disorder in some of the rats. This amount contituted some- 

 what less than the minimum protective supplement to diet No. 

 84, which in itself contains about 86 mg. of phosphorus. Thus it 

 will be noted that a short exposure to sunlight was equivalent to 

 almost doubling the protective dose of phosphate. If the phos- 

 phate content of the diet is adequate, rats do not develop rickets 

 in spite of being kept in the dark throughout the experiment. 

 The effect of sunlight with other dietaries was also studied, and is 

 being continued. 



Discussion. 



As sunlight has a marked effect on the bony development of 

 rats, it is evident that in future in similar nutritional investiga- 

 tions, the light factor will have to be controlled and standardized. 

 It seems probable that some of the irregularities and lack of con- 

 formity observed by investigators in this field may be attributed to 

 keeping the experimental animals under dissimilar intensities of 



