According to nature and origin the dust may be grouped as follows:

Two rows of machines were selected as being best suited to the purposes of our experiments, one row along the center aisle at the east end of the room and the other row along the north window side of the east end. The operation performed on the window row of machines was that of polishing the magazine tubes, and on the center row that of polishing the trigger guards of the rifles. These two particular rows were selected because they presented the most continuous flow of work and also the most continuous of the grinding operations. The Palmer apparatus was placed midway between two adjoining polishing machines, a position as near a machine as could be obtained without interfering with the work of the operator. The standardization of experimental conditions was exceedingly difficult to obtain, owing to the fact that the work in this shop was of an emergency war nature, and interference with the operators was not to be considered.

Table 1 shows in detail the results of our experimental studies in this shop. (See also fig. 2.)

A second series of similar observations was made in another grinding room of factory A, designated as the heavy polishing shop. This room is rectangular in shape, 249 feet long by 39 feet wide, and 13 feet from floor to ceiling. Ventilation is secured by windows only, of which there are 100.

In this shop the larger parts of the rifles are polished, the method of polishing being exactly the same as that employed, and the abrasive wheels being duplicates of those used, on the two rows of machines studied in the cornering and light polishing shop.

Two rows of machines were selected in this room, four sampling positions being chosen on one row and three on the other. At these seven stations samples were taken coincident with high and low U-tube readings.

In Table 2 are given the results of the experimental observations and analyses in the heavy polishing shop. (See fig. 3.)

It will be noted that the observations were made in pairs. Each pair includes, first, one observation under the normal conditions of operation of the exhaust system, and, second, an observation at the same point after a brief interval (5 to 10 minutes), during which the suction head had been reduced by opening doors in the main exhaust duct between the sampling point and the exhaust fan.

In the miners phthisis prevention committee's report it is stated that "Dr. McCrae found that the dust extracted from the lungs of

deceased miners, by a process of acid oxidation, consisted of extremely minute particles. On measurement, it was ascertained that none of the particles were larger than 12 microns in diameter, and

that the great majority of them were less than 1 or 2 microns, or very much smaller in size than a red blood corpuscle.

"These investigations were sufficient to indicate that it is the very fine particles which are the most important factor in the causation of the dis

ease. It is, therefore, very probable that particles over 12 microns in diameter are relatively unimportant, and that only those lying under the limit of 12 microns need be taken into account."

In the light of these conclusions it is evident that the particles which in our study are of greet wit 28

Creuse-producing

ones are fuore de