It was first shown by Gardner (1938a and b) in animal experiments that tridymite appeared to be more fibrogenic than quartz when tested by intravenous injection in rabbits. In fact, all his animals injected with tridymite died by 11 weeks. King, Mohanty, Harrison, and Nagelschmidt (1953a) used intratracheal injections into the lungs of rats, and compared well graded pure samples of quartz and tridymite, with the result that tridymite produced grade 5 fibrosis after nine weeks as against 38 weeks for quartz. The tridymite sample used had been in contact with hydrofluoric acid during purification, and as it had been found in other experiments (King and others, 1953b) that quartz etched with hydrofluoric acid also was more fibrogenic than normal quartz, there remained a suspicion that a trace of hydrofluoric acid had been the cause of the extra activity of tridymite.A new series of experiments was therefore set up with another sample of tridymite which had never been in contact with hydrofluoric acid. This sample was used at three different levels of dosage 25, and 50 mg. per rat), and electro-dialyzed tridymite which was free from any adsorbed ions was also used at the 25 mg. per rat level.The fibrogenic activity of quartz and other materials has in the past been expressed solely on the basis of the degree of maturity of the lesions and the following five grades of fibrosis were used: grade 1, loose reticulin fibres with no collagen formation; grade 2, compact reticulin fibres with or without collagen formation; grade 3, slightly cellular but mostly collagenous; grade 4, wholly collagenous and completely acellular; and grade 5, acellular, collagenous and confluent. 41

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