226 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I consider Root's method to be neither as reproducible nor his patterns as permanent as the results obtained by my method. MR. W. SCaON: Does the size of the hole in the magnesium oxide film vary with the speed of the particles ? THE LECTURER: I really do not know. It depends on droplet size and thickness of coating. If the thickness of the coating is greater than the droplet diameter one will get a hole somewhat larger than the droplet. If it is not, then I believe the droplet is likely to spread on impact and give a hole considerably larger in diameter. MR. A. FOSTER: There appears to be a considerable range in droplet size. Can the largest of these be taken as representation of single particles, or might they be due to coalescence ? TH• L•CTURF•R: If the hole is circular, it is almost certainly due to a single droplet. If it is of a different shape then it may be due to more than one droplet. The correct result can be achieved by interpreting the plate carefully, and the ability to do so grows with practice. DR. W. MITCHELL: As the particles of magnesium oxide are very fine and therefore presumably very reactive, is there a possibility that certain products might react with the magnesium oxide and that the holes produced are then larger than they should be ? Ta• L•CTURER: I have never found this to occur with cosmetic and insecticidal sprays. It may, however, occur with other products. COMPRESSED GASES AS PROPELLANTS FOR COSMETIC PRODUCTS A. HERZKA, B.Sc., A.R.I.C.* A paper delivered before the Society at the Symposium on Pressurized Packs on 14th October 1958. The dffierence between liquefied and compressed gases are described. Reference is also made to the quality factors which are of importance when formulating with compressed gases. NEARLY TWELVE months have elapsed since the publication of a feasible way of dispensing viscous products in non-aerated form from pressurized packsL It must not be assumed, however, that the idea of utilizing nitrogen and other compressed gases as propellants is a new one. Root• has ably reviewed * Research Department, The Metal Box Co. Ltd., Alperton, Middx.

226 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I consider Root's method to be neither as reproducible nor his patterns as permanent as the results obtained by my method. MR. W. SCaON: Does the size of the hole in the magnesium oxide film vary with the speed of the particles ? THE LECTURER: I really do not know. It depends on droplet size and thickness of coating. If the thickness of the coating is greater than the droplet diameter one will get a hole somewhat larger than the droplet. If it is not, then I believe the droplet is likely to spread on impact and give a hole considerably larger in diameter. MR. A. FOSTER: There appears to be a considerable range in droplet size. Can the largest of these be taken as representation of single particles, or might they be due to coalescence ? TH• L•CTURF•R: If the hole is circular, it is almost certainly due to a single droplet. If it is of a different shape then it may be due to more than one droplet. The correct result can be achieved by interpreting the plate carefully, and the ability to do so grows with practice. DR. W. MITCHELL: As the particles of magnesium oxide are very fine and therefore presumably very reactive, is there a possibility that certain products might react with the magnesium oxide and that the holes produced are then larger than they should be ? Ta• L•CTURER: I have never found this to occur with cosmetic and insecticidal sprays. It may, however, occur with other products. COMPRESSED GASES AS PROPELLANTS FOR COSMETIC PRODUCTS A. HERZKA, B.Sc., A.R.I.C.* A paper delivered before the Society at the Symposium on Pressurized Packs on 14th October 1958. The dffierence between liquefied and compressed gases are described. Reference is also made to the quality factors which are of importance when formulating with compressed gases. NEARLY TWELVE months have elapsed since the publication of a feasible way of dispensing viscous products in non-aerated form from pressurized packsL It must not be assumed, however, that the idea of utilizing nitrogen and other compressed gases as propellants is a new one. Root• has ably reviewed * Research Department, The Metal Box Co. Ltd., Alperton, Middx.