Eugen Goldstein was a German physicist who is best known for his experiments involving the behavior of positively charged particles in a gas. In 1886, he discovered the positive ray, a type of particle that is now known as the proton. He was the first to suggest that a beam of positive particles could be used to detect the presence of a gas, and that this could be used to identify the elements that make up that gas. He also proposed the existence of a negatively charged particle, which we now call the electron. His discoveries laid the groundwork for the development of modern particle physics.