I. With the object of producing standard low-temperature burns in animals, and of studying the area of tissue only partly damaged in a burn, a burning iron has been made capable of applying temperatures from 45°-80°C. to the skin; with this the amount of heat and temperature causing skin damage has been studied, and the macroscopic and microscopic damage due to graded temperatures have been delineated.

II. Macroscopic Changes.

1. Graded temperatures of 45°-80°C. have been applied to the skin of shaved, anæsthetised guinea-pigs, and in some cases rats, for times varying from 10 sec. to 6 and 10 min. Observations have been made of the development of erythema, flare, blanching, blueing, heat fixation, incipient blister formation, œdema, and edge wheal, as also upon the later scab formation and rate of epithelium regeneration.

2. Applications of 47°C. up to 6 minutes produce no visible change.

3. At 50°-55°C. applied for 1 minute and over, there is a critical temperature for the development of permanent and irreversible damage; in animals good scab formation occurs after burning at this temperature.

4. After temperatures of 60°-65°C. the epidermis can be peeled off from the exposed area, leaving a punched-out exposed surface area somewhat like the exposed human blister.

5. A temperature of 70°-80°C. for 10-20 seconds produces severe scabbing.

6. Observations of œdema formation have been checked by wet and dry weights of specimens of skin taken 2 hours after heating. At 55°C. there is some change, but this is more definite at 60°C.

7. The observations recorded here are readily reproducible and provide a standard method of burning for experimental work.

III. Microscopic Changes.

1. The histological effects of mild burns of varying intensity and duration have been described in detail. They follow the general picture previously recorded in necrotic changes.

2. Two different types of reaction are described; with milder burns there is cellular disintegration; with more intense burns, heat coagulation.

3. In more intense burns there is a peripheral shell of changes characteristic of burns of lower temperature.

4. Two substances are lost by the epithelium of skin which has been subjected to a burn:

(a) Basophil granules from the cytoplasm.

(b) Nucleoprotein from the nuclei.

These two substances can be identified in the intercellular blister spaces.

5. Collagen fibres change in structure and staining affinity in the more intense burns.

Our thanks are due to the Carnegie Trustees for a personal grant to one of us (R. J. Rossiter); to the Nuffield Committee and Trustees and to the Medical Research Council for grants which have helped the cost of this research, though this has been also much financed from University sources. We are grateful to Dr. M. C. Manifold for help with some of the preliminary observations and to Dr. A. N. Drury, Dr. L. Colebrook, and Mr. E. Rock Carling for their interest. We also wish to thank Mr. Marsden for technical assistance with the histology.