“It is not easy to express the inexpressible,” he answered with a laugh. “Holmes is a little too scientific for my tastes—it approaches to cold-bloodedness. I could imagine his giving a friend a little pinch of the latest vegetable alkaloid, not out of malevolence, you understand, but simply out of a spirit of inquiry in order to have an accurate idea of the effects. To do him justice, I think that he would take it himself with the same readiness. He appears to have a passion for definite and exact knowledge.”

What does alkaloid mean?
And i also don't understand this part
"To do him justice, I think that he would take it himself with the same readiness."

Question

“It is not easy to express the inexpressible,” he answered with a laugh. “Holmes is a little too scientific for my tastes—it approaches to cold-bloodedness. I could imagine his giving a friend a little pinch of the latest vegetable alkaloid, not out of malevolence, you understand, but simply out of a spirit of inquiry in order to have an accurate idea of the effects. To do him justice, I think that he would take it himself with the same readiness. He appears to have a passion for definite and exact knowledge.”

What does alkaloid mean?
And i also don't understand this part
"To do him justice, I think that he would take it himself with the same readiness."

Answer

Alkaloids are an organic compound, so another reading of this could be "a chemical from vegetables." In this context, it seems to refer to vegetable alkaloids being used as medicines/drugs at the time of Sherlock Holmes. Holmes is a scientist and a very curious person, so he would give his friend a vegetable alkaloid as an experiment, but not out of malevolence. "To do him justice" refers to the fact that Holmes is not doing this out of ill-will, but rather that he would be willing to take the vegetable alkaloid himself.

I hope that helps! Let me know if you have any questions.