Автор: Christian Surber

In the early 20th century, tanned skin was associated with good health. However, people began to protect themselves against potential overexposure to avoid sunburns. Around 1945, the first sunscreen …

The concept of expressing acidity as the negative logarithm of the hydrogen ion concentration was defined and termed p H in the beginning of the 20th century. The general usefulness of the p H concep …