Effect of heat.-Heating for 1 hour at 56 degree or 80 degree C. has no effect on the antibacterial power of penicillin. Boiling for a few minutes hardly affects it (see Table II). Boiling for 1 hour reduces it to less than one quarter its previous strength if the fluid is alkaline, but if it is neutral or very slightly acid then the reduction is much less. Autoclaving for 20 minutes at 115 degree C. practically destroys it. Effect of filtration.-Passage through a Seitz filter does not diminish the antibacterial power. This is the best method of obtaining sterile active mould broth. Solubility.-It is freely soluble in water and weak saline solutions. My colleague, Mr. Ridley, has found that if penicillin is evaporated at a low temperature to a sticky mass the active principle can be completely extracted by absolute alcohol. It is insoluble in ether or chloroform. Rate of development of inhibitory substance in culture.-A 500 e.e. Erlenmeyer flask containing 200 e.e. of broth was planted with mould spores and incubated at room temperature (10 degree to 20 degree C.). The inhibitory power of the broth to staphylococcus was tested at intervals. After 5 days complete inhibition in 1 in 20 dilution. Grown at 20 degree C. the development of the active principle is more rapid and a good sample will completely inhibit staphylococci in a 1 in 500 or 1 in 800 dilution in 6 or 7 days. As the culture ages the antibacterial power falls and may in 14 days at 20 degree C. have almost disappeared. The antibacterial power of penicillin falls when it is kept at room temperature. The rate of this fall can be seen from Table I. If the reaction of penicillin is altered from its original pH of 9 to a pH of 6 middot 8 it is much more stable. The small drops of bright yellow fluid which collect on the surface of the mould may have a high antibacterial titre. One specimen of such fluid completely inhibited the growth of staphylococci in a dilution of 1 in 20,000 while the broth in which the mould was growing, tested at the same time, inhibited staphylococcal growth in 1 in 800. If the mould is grown on solid medium and the felted mass picked off and Jack needs to prevent bacterial growth in a solution of nutrient media, and it needs to prevent Staphylococcus contaminants from growing for at least 15 days. a. According to figure 1 of paper, how much Penicillin extract should he add if he needs to make 1600 mL of nutrient media? i. What volume of Penicillin should he add? mL ii. What volume of nutrient media should he add? mL