62 Kansas Academy of Science. 



peculiar conditions was recently afforded the author. In the 

 Chemistry building at the State University, which was completed 

 in 1900, there is a room which is plastered on the sides, and in 

 which is situated the fan which ventilates the building. On one 

 side this room is partially open to the air, and cold air is drawn 

 over hot steam-pipes in the cold weather as it enters the room. 

 The air is then drawn through the fan into the flues, by which, 

 after passing over other steam-coils, it is distributed throughout 

 the building. The temperature of the air in the fan room varies 

 with that of the outside air, ranging probably in the winter from 

 40° F. to 72° F. 



It was noticed that the plastering on both sides of the room, 

 above a line extending from the floor on the sides nearest the 

 pipes to the ceiling on the opposite side, had become powdery, 

 and fallen off. In order, if possible, to flnd the cause of this, sev- 

 eral analyses were made of the hard plaster which was still in place 

 in the wall and of that which had fallen off, with the following re- 

 sults in one case : 



Component parts. '^^ S¥e". 



Silica and insoluble residue 74.41% 76.42% 



Iron and aluminum oxids 2 . 45 1.18 



Magnesium carbonate 1 . 64 1 . 09 



Calcium carbonate 3 . 09 6 . 18 



Calcium sulfate 13.58 12.72 



Water 4.75 1.98 



Totals 99.92 99.57 



Other samples gave : 



Water.' 4.76% 1.57% 



Water 4.56 2.05 



As the samples are taken from different parts of the wall, they will 

 not agree in composition. An inspection of the analysis, however, 

 shows that in every case the fallen plaster contains less than half 

 as much water as the plaster which is in place on the wall. Com- 

 paring this analysis with those previously given, we can also form 

 some estimate of the amount of sand that is used with the original 

 material. 



It is generally conceded that the setting of plaster of Paris is due 

 to interlacing of the small crystals of gypsum, which are formed in 

 the moist material, and we are familiar with the fact that if all the 

 water is driven out of the gypsum in heating, the plaster will not 

 readily "set." It is evident, then, that about five per cent, of water 

 is necessary to the stability of the finished wall. 



