Specific usage of the word 'but'

In the fragment

... If you had but touched me, my friend, you ...

but functions as an adverb whose definition, according to the New Oxford American Dictionary (2nd Edition) is

no more than; only

Thus, your example could very well read

• ... If only you had touched me, my friend, you ...

Other examples:

• I am but a mere mortal.
• He is but a child.
• That was but a distant memory.

This usage of but, though, is largely restricted to formal or literary contexts.

In this case "but" could be replaced with "merely" to give the sense of something small or trivial.

This construction is usually seen when the speaker is reproaching someone for over-reacting when a small action could have prevented a problem. For example,

If you had but (merely) touched me, I would have awakened showing you I was not dead and you would not have called the police.

Or

If you had but (merely) asked, I would have agreed. But you didn't and now we are both disappointed.

But here means only. The phrase then translates to if you had only touched me which means that the hypothesis need not be strengthened for the conclusion to be true.